mod_cache: Fix (max-stale with no '=') and enforce (check integers after '=')

Cache-Control header parsing.

Submitted by: jailletc36
Reviewed by: jim, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1748330 13f79535-47bb-0310-9956-ffa450edef68