title: mqtt message not sent in qos 2 of pub and sub clients
body: <p>Environment</p>
  
  <ul>
  <li>OS: CENTOS 6.5</li>
  <li>EMQ: 2.3.1</li>
  </ul>
  
  <p>PUB client is qos 2 and SUB client is qos 2. But do not subscribe.</p>
  
  <p>SUB Client</p>
  
  <pre><code>import paho.mqtt.client as mqtt
  mqttc = mqtt.Client()
  mqttc.connect("localhost", port=1993, keepalive=60)
  mqttc.subscribe("TEST", 2)
  mqttc.loop_forever()
  </code></pre>
  
  <p>PUB Client</p>
  
  <pre><code>for i in xrange(0, 100):
      print 'i:{}'.format(i)
      payload = {'type': 'TEST', 'raw_data':data}
      (rc, mid) = mqttc.publish("TEST", json.dumps(payload), qos=2)
      print 'i:{} rc:{} mid:{}'.format(i, rc, mid)
      time.sleep(0.3)
  </code></pre>
  
  <p>PUB Client transferring to EMQ is good. But EMQ transferring to SUB Client is broken.</p>
  
  <p>I change PUB Client's qos to 1. The messages are sent to the SUB client.</p>
  
  <p>Why do not transfer to Sub client in qos 2?</p>