mod_cache: Make sure Vary processing handles multivalued Vary headers and
multivalued headers referred to via Vary.

trunk patch: http://svn.apache.org/r1478748

Submitted by: minfrin
Reviewed by: jim, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1487114 13f79535-47bb-0310-9956-ffa450edef68