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.
I am not sure either if this is a good solution. `$pathFilter` is supposed to be a callable (see the section about restricting paths here: http://sabre.io/dav/property-storage/), but currently the `$this->pathFilter($path)` call in https://github.com/DominikTo/sabre-dav/blob/property-storage-fix/lib/DAV/PropertyStorage/Plugin.php#L86 will fail with `Call to undefined method Sabre\DAV\PropertyStorage\Plugin::pathFilter()` as calling $this->pathFilter() in an object context will try to call a method called `pathFilter()` which does not exist.

Another option would be to actually add a method `pathFilter()` and return the property - if it's callable.
This PR is mostly a bug report, because right now the Property Storage plugin doesn't work at all, as the following calls to `$this->pathFilter()` will all fail:

* https://github.com/DominikTo/sabre-dav/blob/property-storage-fix/lib/DAV/PropertyStorage/Plugin.php#L86
* https://github.com/DominikTo/sabre-dav/blob/property-storage-fix/lib/DAV/PropertyStorage/Plugin.php#L103
* https://github.com/DominikTo/sabre-dav/blob/property-storage-fix/lib/DAV/PropertyStorage/Plugin.php#L119
* https://github.com/DominikTo/sabre-dav/blob/property-storage-fix/lib/DAV/PropertyStorage/Plugin.php#L135
* https://github.com/DominikTo/sabre-dav/blob/property-storage-fix/lib/DAV/PropertyStorage/Plugin.php#L138

/cc @evert