Merge r481886 from trunk:

* Remove expired content from cache that cannot be revalidated.

PR: 30370

Submitted by: rpluem
Reviewed by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@572626 13f79535-47bb-0310-9956-ffa450edef68