Merge r161691 from trunk:

 * mod_deflate should be merging the Vary header, not Setting it, and ignoring
   what other modules put there.

Spotted By: Rici Lake
Submitted by: pquerna


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.0.x@371657 13f79535-47bb-0310-9956-ffa450edef68