Set defaults for min/max Zoom in L.mapbox.tileLayer

When overwriting TileLayer options from TileJSON the defaults of min/max Zoom gets overwritten with `undefined` when not set in TileJSON which causes the map to break.

This occurs when a map is initialized with the smallest possible TileJSON
```json
{
  "tilejson": "2.0.0",
  "tiles": [ "http://a.tiles.mapbox.com/v3/examples.map-9ijuk24y/{z}/{x}/{y}.png" ]
}
```