backport r632749 from trunk:

mod_cache: Revalidate cache entities which have Cache-Control: no-cache
set in their response headers.  PR 44511 [Ruediger Pluem]
 


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@637167 13f79535-47bb-0310-9956-ffa450edef68