mod_cache: Explicitly allow cache implementations to cache a 206 Partial

Response if they so choose to do so. Previously an attempt to cache a 206
was arbitrarily allowed if the response contained an Expires or
Cache-Control header, and arbitrarily denied if both headers were missing.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@952823 13f79535-47bb-0310-9956-ffa450edef68