412814 - HttpClient calling CompleteListener.onComplete() twice.

The problem was stemming from the HttpSender completing the response,
but the response was actually completed by HttpReceiver, resulting in the
double call to onComplete().

Now HttpSender checks whether it was able to complete the response, and
only in that case calls the onComplete() callback.
Merged branch 'master' into 'jetty 9.1'.