mod_cache: Make sure that contradictory entity headers present in a 304

Not Modified response are caught and cause the entity to be removed.

trunk patch: http://svn.apache.org/r1479117
2.4.x patch: http://people.apache.org/~minfrin/httpd-mod_cache-304sanity.patch2.4.patch

Submitted by: minfrin
Reviewed by: jim, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1487118 13f79535-47bb-0310-9956-ffa450edef68