Back out r818492 which prevented all caching of incomplete responses.

Instead move the check to mod_disk_cache.  This leaves cache implementations
the flexibility to implement caching of incomplete responses.
PR: 15866


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@821763 13f79535-47bb-0310-9956-ffa450edef68