dav: Fix mounting when 403 is returned for the parent folder

The recent commit e9653aa9, which allows mounting when 403 is returned instead of 401, broke the backend for the case when authentication succeeded for subdirectory and 403 is returned for the parent folder. The backend doesn't work properly, even though it doesn't return any error from the mount operation. Nautilus just shows "File is of unknown type" and the backend prints errors like "soup_auth_authenticate: assertion 'password != NULL' failed". Let's prevent usage of the workaround from commit e9653aa9 after the first successful auth to fix this issue.

Fixes: #417 (closed)