JSONP transport fails when sending JSON stringified message 

JSONP transport fails when sending JSON stringified message, due to unescape mismatch

Code to reproduce the issue

```JavaScript
var socket = eio('ws://localhost:9999', {
  forceJSONP : true
});

socket.on('open', function(){
  socket.send(JSON.stringify({
    a : 'c\nd'
  }));
});
```

An escaped message (e.g. '{a:"c\\\nd"}') does not get escaped by https://github.com/LearnBoost/engine.io-client/blob/master/lib/transports/polling-jsonp.js#L214 but gets unescaped by https://github.com/LearnBoost/engine.io/blob/master/lib/transports/polling-jsonp.js#L46
Resulting in parse error due to invalid payload length and invalid json