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

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.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1591312 13f79535-47bb-0310-9956-ffa450edef68