Merge r327179 from trunk:

* Fix PR31226 (AddOutputFilterByType deflate not active with mod_proxy), by
  allowing ap_add_output_filters_by_type to handle proxied requests.
  Basic tests by jorton and me show that this works, nobody can actually
  remember why this limitation was introduced at all (r94028) and the mailing
  list archives also gave no hint.

Submitted by: Joe Orton


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@327793 13f79535-47bb-0310-9956-ffa450edef68