WebDAV Auth Connector: Check if already logged in

See code comment on what it does.

Basically, we don't need our WebDAV server to do the basic auth handling if the session is already authenticated (by cookie).

So this one will work, given the correct cookies:
```curl -v -X PROPFIND -H "Cookie: oc0b4d3e5b1b=biljubj3n0pnmougco5hddg317"  http://127.0.0.1/remote.php/webdav```

And this one will still ask for HTTP auth:
```curl -v -X PROPFIND http://127.0.0.1/remote.php/webdav```