More mod_cache tweakage.

* modules/cache/cache_storage.c

  (cache_select_url): Add If-Modified-Since regardless of having
    an ETag or not.  See: RFC2616, 14.26.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@152948 13f79535-47bb-0310-9956-ffa450edef68