Merge r1479216 from trunk:

mod_cache: RFC2616 14.9.3 The s-maxage directive also implies the
semantics of the proxy-revalidate directive.

Submitted by: minfrin
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1485670 13f79535-47bb-0310-9956-ffa450edef68