Backport r1669299 from trunk:

 
  *) mod_proxy_wstunnel: Avoid an empty response by failing with 502 (Bad
     Gateway) when no response is ever received from the backend.

Submitted By: jkaluza
Reviewde By: covener, ylavic, trawick




git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1673902 13f79535-47bb-0310-9956-ffa450edef68