mod_cache: Correctly handle HEAD requests on expired cache content. PR 41230.

* modules/cache/mod_cache.c
  (cache_save_filter): Properly handle HEAD responses when we have a stale handle.

(This patch was revised by Justin/Ruediger.)

Submitted by: Niklas Edmundsson
Reviewed by: Justin, Ruediger


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@539620 13f79535-47bb-0310-9956-ffa450edef68