Try to correctly follow RFC 2616 13.3 on validating stale cache

responses by teaching mod_cache's cache_select_url and
cache_save_filter how to deal with this corner case.
PR:
Obtained from:
Submitted by:	jerenkrantz
Reviewed by:	stoddard, jerenkrantz, minfrin, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@105439 13f79535-47bb-0310-9956-ffa450edef68