Fix issue when 'grid.json' format is not captured properly due to a weird behaviour in regex

lib/cartodb/controllers/layergroup.js

@@ -56,7 +56,7 @@ LayergroupController.prototype.register = function(app) {
         this.tile.bind(this));
 
     app.get(app.base_url_mapconfig +
-        '/:token/:z/:x/:y.:format', cors(), userMiddleware,
+        '/:token/:z/:x/:y.(:format)', cors(), userMiddleware,
         this.tile.bind(this));
 
     app.get(app.base_url_mapconfig +