mod_cache: Don't cache requests with a expires date in the past;

otherwise mod_cache will always try to cache the URL. This bug
might lead to numerous rename() errors on win32 if the URL was
previously cached.

+1: minfrin, rpluem, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@463496 13f79535-47bb-0310-9956-ffa450edef68