mod_proxy: Give ap_proxy_post_request as chance to act correctly on the status

mod_proxy: Give ap_proxy_post_request as chance to act correctly on the status
code by setting r->status temporarily to access_status. r->status might be
different than access_status e.g. r->status could be HTTP_OK if e.g. we
override the error page on the proxy or if the error was not generated by the
backend itself but by the proxy e.g. a bad gateway.

Backports: r1597352
Submitted by: rpluem
Reviewed by: wrowe, ylavic

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1750838 13f79535-47bb-0310-9956-ffa450edef68