Change default arguent value for clarity

It is valie to pass None in service_params arguments, so make it the default.

server/lib/python/cartodb_services/cartodb_services/mapzen/isochrones.py

@@ -20,7 +20,7 @@ class MapzenIsochrones:
         "car": "auto"
     }
 
-    def __init__(self, app_key, logger, service_params={}):
+    def __init__(self, app_key, logger, service_params=None):
         service_params = service_params or {}
         self._app_key = app_key
         self._logger = logger