Backport from 2.1.

  *) mod_cache: Fix the cache code so that responses can be cached
     if they have an Expires header but no Etag or Last-Modified
     headers. PR 23130.
     [bjorn@exoweb.net]

Reviewed by: Paul J. Reder, Roy T. Fielding, Brian Pane


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@101326 13f79535-47bb-0310-9956-ffa450edef68