When the server closes a connection, make sure the send buffer still gets flushed.

On polling protocols, you otherwise do not actually get the final output from the server.  eg:

*server*
```js
var closer = sockjs.createServer();
closer.on('connection', function(conn) {
  conn.write("first message");
  conn.close();
});
```

*client*
```js
   var sock = new SockJS('http://127.0.0.1:9999/closer', undefined, {
        protocols_whitelist: [
           'xhr-polling'
        ]});
   sock.onopen = function() {
       console.log('open');
   };
   sock.onmessage = function(e) {
       console.log('message', e.data);
   };
   sock.onclose = function() {
       console.log('close');
   };
```

Without this patch, the client just logs open and close.  With this patch, the client logs the message as well.