mod_proxy: enable absolute URLs to be rewritten with ProxyPassReverse,

e.g. to reverse proxy "Location: https://other-internal-server/login" with
ProxyPassReverse https://public-address/ https://other-internal-server/



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1152273 13f79535-47bb-0310-9956-ffa450edef68