mod_cache: Ensure that updated responses to HEAD requests don't get

mistakenly paired with a previously cached body. Ensure that any existing
body is removed when a HEAD request is cached.

trunk patch: http://svn.apache.org/r1479411

Submitted by: minfrin
Reviewed by: jim, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1487122 13f79535-47bb-0310-9956-ffa450edef68