More tweaks primarily driven by caching.

* modules/http/http_protocol.c

  (ap_meets_conditions): Allow If-None-Modified and If-Modified-Since
   to interact as described in RFC2616, sections 14.26 and 13.3.4.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@153270 13f79535-47bb-0310-9956-ffa450edef68