Fixed mod_expires: Expires time shouldn't be in the past.

r1002205 in test framework needs to be reverted now since this is fixed.
Author: rjung, reviewed by: wrowe, sf.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.0.x@1004974 13f79535-47bb-0310-9956-ffa450edef68