I have been using AddParameter to include XML bodies in my HTTP requests:
request.AddParameter(contentType, body, ParameterType.RequestBody);
>
Modifications to Http.Async.cs are also necessary for the RequestStreamCallback method. I'm actually working on getting this fix into the repo and published to Nuget as I'm helping to maintain the project now. Here's a link to the issue that's been created for this: https://github.com/restsharp/RestSharp/issues/583