I serve map png files from disk and I have tile pngs for whole city in zoom level 15. I have also tiles in zoom levels 16-18 but only for certain areas.

I would like to set up the tile Layer, so that when the user is in zoom level 18 the map will display scaled tiles from level 15 as a fallback.

I tried setting option `maxNativeZoom`, but didn't work for me.

Here is my code:

```
offlineLayer = L.mapbox.tileLayer(tileJSON, {
    minZoom: 8,
    maxZoom: 18,
    maxNativeZoom: 15
});
map.addLayer(offlineLayer, 'Offline', 1);

```

Can I make it work, that way using some options or do I need to hack it some way? Or is there some example code for that?