Merge r580466 from trunk:

Fix keepalive to proxy backend
PR 43472

Submitted by: rpluem
Reviewed by: rpluem, niq, trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@582620 13f79535-47bb-0310-9956-ffa450edef68