mod_dav providers define a 'can_be_activity' callback. Unfortunately,

mod_dav providers define a 'can_be_activity' callback.  Unfortunately,
mod_dav isn't calling it before creating an activity.  This is a
required precondition (along with the resource not existing), as
defined in the deltaV RFC (3253), section 13.5.

* mod_dav.c (dav_method_make_activity): if available, call provider's
  'can_be_activity' callback as a precondition to making an activity.

Submitted by: Ben Collins-Sussman <sussman@apache.org>
Reviewed by: Sander Striker

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@99966 13f79535-47bb-0310-9956-ffa450edef68