Grails RestfulController doesn't respond with a JSON when Content-Type: application/x-www-form-urlencoded header is present

I'm trying to implement a simple RestfulController for my application. Given the following domain class:

class Test {
    String name
    int someInteger

    static constraints = {
    }
}

and its controller:

class TestController extends RestfulController<Test>{
    TestController() {
        super(Test)
    }
}

Inside conf/UrlMappings.groovy I added the following entries:

"/api/$controller?(.${format})?" {
    action = [POST: "save", PUT: "save", GET: "index", DELETE:"error"]
}

"/api/$controller/$id?(.${format})?" {
    action = [POST: "update", PUT: "update", GET: "show", DELETE: "delete"]
}

Get requests are working fine, but Post and Put requests to a URL like http://localhost:8080/app/api/test.json when the Content-Type: application/x-www-form-urlencoded Header is present fail to respond with a JSON as expected. Instead render the show action view after persisting the entrie sent.

I also tried to use the Header Accept: application/json with no effect.

How can I fix that?

Edit:

Further investigating RestfulController's source file and the docs section regarding Content Negotiation I was able fix it by overriding the save and update methods replacing the line:

request.withFormat {

with:

withFormat {

Is it intentional or is there a flaw on RestfulController's implementation? Why does it consider the Content-Type header instead of the Accept header to render response?

Sorry for taking so long to respond. I had some trouble putting everything to work. Thanks a lot @Dónal for all the help. Ended using the following class to do the trick:

import org.codehaus.groovy.grails.web.servlet.HttpHeaders;
import org.springframework.http.HttpStatus;

import grails.artefact.Artefact;
import grails.rest.RestfulController;
import grails.transaction.Transactional;

@Artefact("Controller")
@Transactional(readOnly = true)
class MyRestfulController<T> extends RestfulController<T> {

    public MyRestfulController(Class<T> resource, boolean readOnly = false) {
        super(resource, readOnly);
    }

    @Override
    @Transactional
    def save() {
        if(handleReadOnly()) {
            return
        }
        T instance = createResource(getParametersToBind())

        instance.validate()
        if (instance.hasErrors()) {
            respond instance.errors, view:'create' // STATUS CODE 422
            return
        }

        instance.save flush:true

        def formatHolder = params.format ? this : request
        formatHolder.withFormat {
            form multipartForm {
                flash.message = message(code: 'default.created.message', args: [message(code: "${resourceName}.label".toString(), default: resourceClassName), instance.id])
                redirect instance
            }
            '*' {
                response.addHeader(HttpHeaders.LOCATION,
                        g.createLink(
                                resource: this.controllerName, action: 'show',id: instance.id, absolute: true,
                                namespace: hasProperty('namespace') ? this.namespace : null ))
                respond instance, [status: HttpStatus.CREATED]
            }
        }
    }

    @Override
    @Transactional
    def update() {
        if(handleReadOnly()) {
            return
        }

        T instance = queryForResource(params.id)
        if (instance == null) {
            notFound()
            return
        }

        instance.properties = getParametersToBind()

        if (instance.hasErrors()) {
            respond instance.errors, view:'edit' // STATUS CODE 422
            return
        }

        instance.save flush:true

        def formatHolder = params.format ? this : request
        formatHolder.withFormat {
            form multipartForm {
                flash.message = message(code: 'default.updated.message', args: [message(code: "${resourceClassName}.label".toString(), default: resourceClassName), instance.id])
                redirect instance
            }
            '*'{
                response.addHeader(HttpHeaders.LOCATION,
                        g.createLink(
                                resource: this.controllerName, action: 'show',id: instance.id, absolute: true,
                                namespace: hasProperty('namespace') ? this.namespace : null ))
                respond instance, [status: HttpStatus.OK]
            }
        }
    }

}

By using def formatHolder = params.format ? this : request and then call formatHolder.withFormat I am now able to override the response format independently from the request format.

It doesn't work for the Accept Header yet but at least it works.