How to convert x-www-form-urlencoded post Message to JSON post Message?

I have a requirement to support data being posted to our WCF service with a content type of x-www-form-urlencoded. Since WCF doesn't like to do that natively, the thought I had was to use a MessageInspector to intercept incoming messages with that content type, read out the body, convert it to a JSON string, and replace the request message.

Problem is that I can't seem to make a new Message object that my service actually likes. I can get the body and turn it into a JSON string just fine, but the new message I create causes errors instead of going on through to the appropriate service method.

This is what I've got currently. I've been tinkering with this for a day and a bit now, trying a few different ways, but have had little luck. I'll post below the error I'm currently getting.

Web service method I want to call:

[OperationContract]
[WebInvoke(Method = "POST", UriTemplate = "/PostTest", ResponseFormat = WebMessageFormat.Json, RequestFormat = WebMessageFormat.Json)]
public string PostTest(TestObject thinger)
{
    return thinger.Thing;
}

The message inspector:

public class FormPostConverter : IDispatchMessageInspector
{
    public object AfterReceiveRequest(ref Message request, IClientChannel channel, InstanceContext instanceContext)
    {
        var contentType = (request.Properties[HttpRequestMessageProperty.Name] as HttpRequestMessageProperty)?.Headers["Content-Type"];

        if (!request.IsEmpty && contentType == "application/x-www-form-urlencoded")
        {
            var body = HttpUtility.ParseQueryString(new StreamReader(request.GetBody<Stream>()).ReadToEnd());
            var json = new JavaScriptSerializer().Serialize(body.AllKeys.ToDictionary(k => k, k => body[k]));

            Message newMessage = Message.CreateMessage(MessageVersion.None, "", json, new DataContractJsonSerializer(typeof(string)));
            newMessage.Headers.CopyHeadersFrom(request);
            newMessage.Properties.CopyProperties(request.Properties);
            (newMessage.Properties[HttpRequestMessageProperty.Name] as HttpRequestMessageProperty)?.Headers.Set(HttpRequestHeader.ContentType, "application/json");
            newMessage.Properties[WebBodyFormatMessageProperty.Name] = new WebBodyFormatMessageProperty(WebContentFormat.Json);
            request = newMessage;
        }

        return null;
    }

    public void BeforeSendReply(ref Message reply, object correlationState)
    { }
}

The error I'm getting:

Request Error The server encountered an error processing the request. The exception message is 'Expecting state 'Element'.. Encountered 'Text' with name '', namespace ''. '.