Merge r539621 from trunk:

mod_cache: Do not set Date or Expires when they are missing from the original
response or are invalid.

Submitted by: jerenkrantz
Reviewed by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@553600 13f79535-47bb-0310-9956-ffa450edef68