fixed GridLayer not showing up after removing and adding the same instance

Commit e09397929798298983e73d5e148bb2ceed0a5fba introduced a bug causing a GridLayer (in this example TileLayer) not showing up anymore after removing and adding the same instance.

So basically the following will cause a TileLayer to be hidden (and only show itself again after e.g., zooming)
```javascript
map.removeLayer(tileLayer);
map.addLayer(tileLayer);
```

I've fixed the bug by just resetting the ```this._tileZoom``` after removing the layer from the map.

The included debug page will show the fix (clicking the button will remove the layer and one second after that add it again).