Json message (#214)

If the MQTT payload is a string the AWS MQTT test client shows a "Message cannot be displayed in specified format." warning.

Description of changes:

Changed the payload to json to prevent the aforementioned warning/error.

Author: sbSteveK

samples/pubsub.py

@@ -8,6 +8,7 @@
 import threading
 import time
 from uuid import uuid4
+import json
 
 # This sample uses the Message Broker for AWS IoT to send and receive messages
 # through an MQTT connection. On startup, the device connects to the server,
@@ -155,9 +156,10 @@ def on_message_received(topic, payload, dup, qos, retain, **kwargs):
         while (publish_count <= args.count) or (args.count == 0):
             message = "{} [{}]".format(args.message, publish_count)
             print("Publishing message to topic '{}': {}".format(args.topic, message))
+            message_json = json.dumps(message)
             mqtt_connection.publish(
                 topic=args.topic,
-                payload=message,
+                payload=message_json,
                 qos=mqtt.QoS.AT_LEAST_ONCE)
             time.sleep(1)
             publish_count += 1