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 think we should change the callsite's so PHP don't need to guess if we like to call a property or a method.
just introduce a local var and you are good to go.

Instead of
```
if ($this->pathFilter && !$this->pathFilter($path)) return;
```

just do
```
$pathFilter = $this->pathFilter;
if ($pathFilter && !$pathFilter($path)) return;
```

would make it obious I think. this could of course be done in a private method, so this hack won't be distributed across the whole class..

WDYT?