mod_disk_cache: Allow Vary'd content to be refreshed properly.

(Backport of r538992, r538997, r539054)

Reviewed by: jerenkrantz, fielding, rpluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@539279 13f79535-47bb-0310-9956-ffa450edef68