mod_cache: When serving from cache, only the last header of a multivalued

header was taken into account. Fixed. Ensure that Warning headers are correctly
handled as per RFC2616.

trunk patch: http://svn.apache.org/r1478441
             http://svn.apache.org/r1480283
2.4.x patch: http://people.apache.org/~minfrin/httpd-mod_cache-multipleheaders2.4.patch

Submitted by: minfrin
Reviewed by: jim, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1487108 13f79535-47bb-0310-9956-ffa450edef68