Merge this fix from 2.1-dev:

*) mod_expires: Initialize ExpiresDefault to NULL instead of "" to
   avoid reporting an Internal Server error if it is used without
   having been set in the httpd.conf file. PR: 23748, 24459

   Reviewed by: Andre Malo and Jeff Trawick.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@101890 13f79535-47bb-0310-9956-ffa450edef68