Revision 1445103

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