title: How to publish a protobuf message to mqtt topic
body: <p>I can get a string by <code>protobuf.SerializeToString()</code> methond, but when I call <code>mqttclient.publish(topic, protobuf.SerializeToString(), QoS)</code>, I got the following error:</p>
  <pre><code>TypeError: payload must be a string, bytearray, int, float or None.
  </code></pre>
  <p>I think <code>protobuf.SerializeToString()</code> returns a string, why I got the error message.</p>