More work to properly handle revalidated responses correctly.

* modules/cache/mod_cache.c: If we add a new Expires value, tell our client;
  merge in headers properly (or better than before) so that we can update the
  header fields on a revalidated but with updated header fields.
* modules/cache/mod_cache.h, modules/cache/cache_storage.c:
  Add preserve_orig flag to ap_cache_accept_headers to allow updating of fields.
* modules/cache/mod_disk_cache.c: Load status value from disk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@152973 13f79535-47bb-0310-9956-ffa450edef68