[SOLVED] Ionic 4 native http POST with multiple headers

Here is how I am doing an http post from my Ionic app:

import { HTTP } from '@ionic-native/http/ngx';

someAction(assetId: number, userId: number)
{
    var asset: Asset = new Asset();
    asset.assetId = assetId.toString();
    asset.userId = userId.toString();

    const options = {
       headers:  "Content-Type: application/json; charset=utf-8"
    };

    return this.http.post(this.url + "someAction", asset, options).then(data => {
        return "Done";
    }).catch(err => {
        console.log(err);            
    });
}

According to the documentation here - https://www.npmjs.com/package/cordova-plugin-advanced-http#post, the headers should be sent like so:

headers: { 'content-length': '247' }

Which is not working for me. Note how I am sending the header key and value as one string:

headers: "Content-Type: application/json; charset=utf-8"

This is the error -

Error: advanced-http: header values must be strings at checkKeyValuePairObject (helpers.js:88) at checkHeadersObject (helpers.js:192) at Object.handleMissingOptions (helpers.js:309) at Object.sendRequest (public-interface.js:143) at Object.post (public-interface.js:167) at callCordovaPlugin (vendor.js:76774) at vendor.js:76589 at vendor.js:76547 at new ZoneAwarePromise (polyfills.js:3268) at tryNativePromise (vendor.js:76546)

I get the same error when trying to send multiple headers. I am at a loss as to why this is happening - the example in the documentation does not work! What am I missing?

Edit: web.config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
    <configSections>
    <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />
</configSections>
<connectionStrings>
    <add name="Default" connectionString="..." />
</connectionStrings>

<appSettings>
    ...
</appSettings>

<system.web>
    <compilation debug="true" targetFramework="4.5.2" />
    <httpRuntime targetFramework="4.5.2" maxRequestLength="75096" executionTimeout="600" />
</system.web>

<runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">

    <dependentAssembly>
        <assemblyIdentity name="Microsoft.Owin" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-4.0.0.0" newVersion="4.0.0.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="System.IdentityModel.Tokens.Jwt" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-5.2.2.0" newVersion="5.2.2.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="Microsoft.IdentityModel.Protocol.Extensions" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-1.0.40306.1554" newVersion="1.0.40306.1554" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="Microsoft.Owin.Security" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-3.1.0.0" newVersion="3.1.0.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="Microsoft.Owin.Security.OAuth" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-3.1.0.0" newVersion="3.1.0.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="Microsoft.Owin.Security.Jwt" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-3.1.0.0" newVersion="3.1.0.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="Microsoft.IdentityModel.Tokens" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-5.2.2.0" newVersion="5.2.2.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="System.Web.Http" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-5.2.6.0" newVersion="5.2.6.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="System.Net.Http.Formatting" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-5.2.6.0" newVersion="5.2.6.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="Autofac" publicKeyToken="17863af14b0044da" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-4.8.1.0" newVersion="4.8.1.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="System.Web.Http.WebHost" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-5.2.6.0" newVersion="5.2.6.0" />
     </dependentAssembly>
     <dependentAssembly>
        <assemblyIdentity name="System.Web.Mvc" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-5.1.0.0" newVersion="5.1.0.0" />
    </dependentAssembly>

    <dependentAssembly>
        <assemblyIdentity name="Newtonsoft.Json" publicKeyToken="30ad4fe6b2a6aeed" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-11.0.0.0" newVersion="11.0.0.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="System.Web.WebPages" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-3.0.0.0" newVersion="3.0.0.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="System.Web.Razor" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-3.0.0.0" newVersion="3.0.0.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="System.Web.WebPages.Razor" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-3.0.0.0" newVersion="3.0.0.0" />
    </dependentAssembly>
    <dependentAssembly>
        <assemblyIdentity name="WebGrease" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-1.5.2.14234" newVersion="1.5.2.14234" />
    </dependentAssembly>
</assemblyBinding>
</runtime>

<system.codedom>
    <compilers>
        <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701" />
        <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+" />
    </compilers>
</system.codedom>

<system.webServer>

<validation validateIntegratedModeConfiguration="false" />
<modules runAllManagedModulesForAllRequests="true">
  <remove name="FormsAuthentication" />
  <remove name="WebDAVModule" />
</modules>

<handlers>
  <remove name="WebDAV" />
  <remove name="ExtensionlessUrlHandler-ISAPI-4.0_32bit" />
  <remove name="ExtensionlessUrlHandler-ISAPI-4.0_64bit" />
  <remove name="ExtensionlessUrlHandler-Integrated-4.0" />
  <add name="ExtensionlessUrlHandler-ISAPI-4.0_32bit" path="*." verb="GET,HEAD,POST,DEBUG,PUT,DELETE,PATCH,OPTIONS" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
  <add name="ExtensionlessUrlHandler-ISAPI-4.0_64bit" path="*." verb="GET,HEAD,POST,DEBUG,PUT,DELETE,PATCH,OPTIONS" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
  <add name="ExtensionlessUrlHandler-Integrated-4.0" path="*." verb="GET,HEAD,POST,DEBUG,PUT,DELETE,PATCH,OPTIONS" type="System.Web.Handlers.TransferRequestHandler" preCondition="integratedMode,runtimeVersionv4.0" />
</handlers>

<security>
  <requestFiltering>
    <verbs allowUnlisted="false">
      <clear />
      <add verb="GET" allowed="true" />
      <add verb="POST" allowed="true" />
      <add verb="DELETE" allowed="true" />
      <add verb="PUT" allowed="true" />
      <add verb="OPTIONS" allowed="true" />
    </verbs>
  </requestFiltering>
</security>
<httpProtocol>
  <customHeaders>
    <add name="Access-Control-Allow-Origin" value="*" />
  </customHeaders>
</httpProtocol>

</system.webServer>

<log4net>
...
</log4net>

</configuration>