Fixed Tilemap.removeTile issue to put tile of index -1 instead of null.

Author: draklaw

src/tilemap/Tilemap.js

@@ -1080,7 +1080,7 @@ Phaser.Tilemap.prototype = {
             {
                 var tile = this.layers[layer].data[y][x];
 
-                this.layers[layer].data[y][x] = null;
+                this.layers[layer].data[y][x] = new Phaser.Tile(this.layers[layer], -1, x, y, this.tileWidth, this.tileHeight);
 
                 this.layers[layer].dirty = true;