mod_cache: Let Cache-Control max-age set the expiration of the cached

mod_cache: Let Cache-Control max-age set the expiration of the cached
representation if Expires is not set.

* modules/cache/mod_cache.c
  (cache_save_filter): If Cache-Control max-age is set and Expires isn't,
  let that value control our expiration.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@539063 13f79535-47bb-0310-9956-ffa450edef68