Some WebDAV servers don't implement section 7.3 of RFC4918:
<http://tools.ietf.org/html/rfc4918#section-7.3>

This lack of implementation breaks 'Save As...' functionality
when the target is a WebDAV server, by not locking the URL
('reserve the name for use', in RFC4918 parlance).

The server not implementing this usually answers with one of
'405 Method Not Allowed', '501 Not Implemented' or
'412 Precondition Failed' http error codes.

The fix should manage this lack of implementation.

Change-Id: Ie4689a076aafa365106d02b3ea16459a28fcde65
Reviewed-on: https://gerrit.libreoffice.org/20600Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>