Updating a Contentful entry

I've hit a brick wall whilst attempting to update a Contentful entry using a typical http request in Javascript; I receive the error code "VersionMismatch" which, according to the documentation, means:

This error occurs when you're trying to update an existing asset, entry or content type, and you didn't specify the current version of the object or specify an outdated version.

However, I have specified the current version of the entry using the 'X-Contentful-Version' header parameter as per the documentation, and have used the dynamic property value from 'entry.sys.revision' as the parameter value (as well as hardcoding the current version, plus a bunch of different numbers, but I always receive the same error). This post reported the exact same issue, but was seemingly resolved by adding this header parameter.

Here's my current code, that is also using the Contentful API to retrieve entries from my Contentful space, but I'm having to use plain Javascript to put the data back due to specific requirements:

var client = contentful.createClient(
    {
        space: space_id,
        accessToken: client_token
    }
);

client.getEntry(entry_id).then((entry) => entry).then(function(entry) {

    // update values of entry
    entry.fields.title = "Testing";

    // post data to contentful API
    var request = new XMLHttpRequest();
        request.open('PUT', 'https://api.contentful.com/spaces/' + space_id + '/entries/' + entry_id);
        request.setRequestHeader('Authorization', 'Bearer my_access_token');
        request.setRequestHeader('Content-Type', 'application/vnd.contentful.management.v1+json');
        request.setRequestHeader('X-Contentful-Content-Type', entry.sys.contentType.sys.id);

        // setting the 'X-Contentful-Version' header with current/soon to be old version
        request.setRequestHeader('X-Contentful-Version', entry.sys.revision);

    // convert entry object to JSON before sending
    var body = JSON.stringify({fields: entry.fields});

    request.send(body);
});

Contentful developer here.

It looks like you get your content with the Contentful Delivery SDK and then try yo use that data to update content. That will not work. Instead I recommend using the Contentful Management SDK which will take care of all the versioning header for you.