Allow for "magic" scheme "auto" which makes the scheme of
the backend worker match whatever the scheme of the
incoming request was...

For example:

   ProxyPass / auto://foo.example.com/

If the incoming request is http:.../lala then
the resultant will be http://foo.example.com/lala

If it's wws:.../lolo then we'd send
wws://foo.example.com/lolo

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1602523 13f79535-47bb-0310-9956-ffa450edef68