mod_cache: When content is served stale, and there is no means to

revalidate the content using ETag or Last-Modified, and we have
mandated no stale-on-error behaviour, stand down and don't cache.
Saves a cache write that will never be read.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1103494 13f79535-47bb-0310-9956-ffa450edef68