dav: Try authenticate again even if 403 was returned

Some servers e.g. davs://webdav.mc.gmx.net returns 403 status code in case if invalid credentials instead of 401. This causes that libsoup fails immediately after the first login attempt, which is especially problem in case of the wrong credentials stored in the keyring. A user doesn't have any chance to manually specify correct credentials. Clear the credentials cache and force libsoup to call the authentication callback again...

Closes: #351 (closed)