mod_cache: Check the request to determine whether we are allowed

to return cached content at all, and respect a "Cache-Control:
no-cache" header from a client. Previously, "no-cache" would
behave like "max-age=0".


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@992625 13f79535-47bb-0310-9956-ffa450edef68