title: socket.io - disconnect event doesn't fire in xhr-polling
body: <p>In socket.io the disconnect event doesn't fire when the transport xhr-polling is active.
  If I switch the transport to websockets it works fine, but in xhr-polling its doesn't work.</p>
  
  <pre><code>/* Basics */
  var express = require('express'),
      app = express(),
      server = require('http').createServer(app),
      io = require('socket.io').listen(server);
  
  server.listen(1337, null);
  
  io.set('transports', ['xhr-polling']);
  
  // routing
  app.get('/', function (req, res) {
      res.sendfile("index.html");
      app.use(express.static(__dirname));
  });
  
  io.sockets.on('connection', function (socket) 
      socket.on('disconnect', function() {
          console.log('LOL');
      });
  });
  </code></pre>
  
  <p>In the following code the disconnect doesn't fire, but if I delete the line - <br />
  <code>io.set('transports', ['xhr-polling']);</code> <br />
  It works perfectly, so why it doesn't work with xhr-polling? but only with websockets?
  <br /><br />
  How can I fix this? Am I missing something? </p>
  
  <p>Thanks in Advance ;)</p>