mod_cache: Ensure that we don't attempt to replace a cached response
with an older response as per RFC2616 13.12.

trunk patch: http://svn.apache.org/r1479966
2.4.x patch: http://people.apache.org/~minfrin/httpd-mod_cache-olderdate2.4.patch

Submitted by: minfrin
Reviewed by: jim, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1487129 13f79535-47bb-0310-9956-ffa450edef68