Merge r645813 from trunk:

* In the case that we fail to read the response line from the backend and if
  we are a reverse proxy request shutdown the connection WITHOUT ANY response
  to trigger a retry by the client if allowed (as for idempotent requests).
  BUT currently we should not do this if the request is the first request on
  a keepalive connection as browsers like seamonkey only display an empty page
  in this case and do not do a retry.

Related to PR 37770

Submitted by: rpluem
Reviewed by: rpluem, jim, fielding


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@657443 13f79535-47bb-0310-9956-ffa450edef68