Found the solution

I asked my question on the asp.net forum and I got the answer. Go here to see the solution.

Anyway, I will include the answer here just in case the post disappeared in the future.

public class ReadAsSingleObjectPolicy : IRequestContentReadPolicy
{
  public RequestContentReadKind GetRequestContentReadKind(HttpActionDescriptor actionDescriptor)
  {
    return RequestContentReadKind.AsSingleObject;
  }
}

GlobalConfiguration.Configuration.ServiceResolver
  .SetService(typeof(IRequestContentReadPolicy), new ReadAsSingleObjectPolicy());