title: socketio connects with polling after a websocket connection is established. blocks communication
body: <p>I've been struggling with a strange problem with my websocket connection.
  I use socketio 1.4.5 to connect to the server (flask, flask-socketio, gunicorn, nginx).</p>
  
  <p>The problem is that about 1 of 8 connection attempts socketio connects using xhr polling <strong>after</strong> the websocket connection is established. Somehow this blocks the whole communication for this specific client. When timeout is reached (60seconds in my case) socketio connects again and if no xhr-polling is blocking the next time, the communication is up again.</p>
  
  <p>This i very frustrating. and i have never seen this problem earlier on application with the same setup.</p>
  
  <pre><code>?EIO=3&amp;transport=websocket&amp;sid=997d6d19758641249e3b1232341e9fe2 GET 101 websocket   localhost       websocket   Other   0 B 1.0 min 
  
  ?EIO=3&amp;transport=polling&amp;t=LEUZRW7&amp;sid=997d6d19758641249e3b1232341e9fe2 GET 504 http/1.1    localhost   127.0.0.1:443   xhr socket.io-1.4.5.js:1    763 B   1.0 min
  </code></pre>
  
  <p>The two lines above is what is available on the network tab when analyzing the trafic in chrome. Before the timeout is reached, 504 is replaced with "pending".</p>
  
  <p>I have been trying to find the solution for this for a couple of hours now, and running out of ideas. I hope someone can help me with this..</p>