Update to SabreDAV 2.1, add more tests

Add more unit/integration tests, and upgrade to SabreDAV 2.1, the latest and greatest.
It may be possible to refactor this to:

```php
$propFind->handle('{urn:schemas-microsoft-com:}Win32FileAttributes', function() use ($node) {
    if (method_exists($node, 'getWin32Props')) {
        return $node->getWin32Props();
    }
});
```