Merge r1591320, r1591322, r1591390, r1591394 from trunk:

mod_cache: Don't add cached/revalidated entity headers to a 304 response.
           PR 55547.

When the conditional request meets the conditions of the stale then revalidated
entry, the forwarded 304 response includes the entity headers merged from the
cached headers (before updating the entry).
Strip them before returning a 304.

Since the entity headers are stripped elsewhere, factorize the code using a
new table (MOD_CACHE_ENTITY_HEADERS[]) containing these headers's names.


mod_cache: follow up to r1591320.

Use the new MOD_CACHE_ENTITY_HEADERS[] names to check 304 contradictions
against the same headers.


mod_cache: follow up to r1591322

Avoid one unnecessary test when checking 304 contradictions.


mod_cache: follow up to r1591390

Fix code typo.

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1595916 13f79535-47bb-0310-9956-ffa450edef68