Merge r1573626 from trunk:

mod_proxy: Allow reverse-proxy to be set via explicit handler.

Submitted by: ryo takatsuki <ryotakatsuki gmail com>
Reviewed by: ylavic, jim, mrumph


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1604378 13f79535-47bb-0310-9956-ffa450edef68