Merge r1518269 from trunk:

Expires in a 304 is not a contradiction in terms of r1487118:

     - Expires, Cache-Control, and/or Vary, if the field-value might
        differ from that sent in any previous response for the same
        variant



Submitted by: covener
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1560093 13f79535-47bb-0310-9956-ffa450edef68