Merge pull request GoogleCloudPlatform#1313 from nectarine/iot/end_to_end_example

fix message.payload str casting.

Author: gguuss

iot/api-client/end_to_end_example/cloudiot_pubsub_example_mqtt_device.py

@@ -121,7 +121,7 @@ def on_subscribe(self, unused_client, unused_userdata, unused_mid,
 
     def on_message(self, unused_client, unused_userdata, message):
         """Callback when the device receives a message on a subscription."""
-        payload = str(message.payload)
+        payload = message.payload
         print('Received message \'{}\' on topic \'{}\' with Qos {}'.format(
             payload, message.topic, str(message.qos)))