Edited Issue: HttpControllerHandler.ConvertRequest throws in cases where the request stream has previously been read [1659]

In 5.1, we changed ConvertRequest to use Request.GetBufferedInputStream in lieu of Request.InputStream for reading the content. However the GetBuffer*InputStream methods only works correctly when the stream had not been previously read (for instance by the trace module). HttpRequest.ReadEntityBodyMode can be used to determine if the stream has been previously accessed.