Resolved two issues

* DVPL-4522: Receiver.SendAsync(stream) get timeout error
Partially a test problem and partially an implementation issue: Splunk
returns HTTP status code 204, no content, not HTTP status code 200 as
advertised in the REST API documentation. I notified the documentation
team about this and expect them to correct the POST receivers/stream
page.
* DVPL-4645: ReadMessageAsync() can't handle Response "204: No content"
Discovered and corrected this problem when debugging the
Receiver.SendAsync(Stream) method.
Some code tidying...