mod_cache: Explicitly allow cache implementations to cache a 206 Partial

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.

Currently the disk and memory cache providers do not cache 206 Partial
Responses.

Backport of r952823 from trunk.

Submitted by: minfrin
Reviewed by: rjung, wrowe
Backported by: rpluem

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1445103 13f79535-47bb-0310-9956-ffa450edef68