PropertyStorage fix

Methods and properties are in a different namespace, so currently $this->pathFilter() is an undefined method which causes:

```
Call to undefined method Sabre\DAV\PropertyStorage\Plugin::pathFilter() in /opt/foo/bar/vendor/sabre/dav/lib/DAV/PropertyStorage/Plugin.php on line 68
```

This intercepts calls to inaccessible method and returns the callable instead.
That indeed seems to be a better solution. Also added a test now.
Let's see what @evert thinks. :)