Merge r1532816 from trunk:

Interpolation worked within the scheme portion of URLs for
ProxyPass and ProxyPassReverse until a fix to apr_uri_parse()
in APR-Util 1.5.2 closed the hole that had previously allowed
the necessary {} characters within the scheme.

Lack of support for interpolation within the scheme will be a
permanent limitation.

The mod_proxy documentation is updated to provide an alternate
configuration solution.

PR: 55315
Submitted by: Mike Rumph <mike.rumph oracle.com>
Tweaked by: trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1532824 13f79535-47bb-0310-9956-ffa450edef68