Fix issue 3515, DAV clients cannot refresh LOCK.

* subversion/mod_dav_svn/lock.c
  (get_locks, has_locks): Make the 'Big Lie' only apply to Subversion
   clients, other clients get standard WebDAV behaviour.

* subversion/tests/cmdline/lock_tests.py
  (dav_lock_refresh): Remove XFAIL.

Patch by: rhuijben


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1584592 13f79535-47bb-0310-9956-ffa450edef68