mod_cache: Preserve non-cacheable headers forwarded from an origin 304

           response. PR 55547.

When mod_cache asks for a revalidation of a stale entry and the origin responds
with a 304 (not that stale), the module strips the non-cacheable headers from
the origin response and merges the stale headers to update the cache.

The problem is that mod_cache won't forward the non-cacheable headers to the
client, for example if the 304 response contains both Set-Cookie and
'Cache-Control: no-cache="Set-Cookie"' headers, or CacheIgnoreHeaders is used.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1591328 13f79535-47bb-0310-9956-ffa450edef68