mod_dav: Allow other modules to become providers and add resource types

to the DAV response.
Submitted by: Jari Urpalainen <jari.urpalainen nokia.com>, Brian France <brian brianfrance.com>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@816893 13f79535-47bb-0310-9956-ffa450edef68