Allow custom socket.io querystring args

This is useful for CSRF protection in custom handshake handlers.

You can set query strings on the server like this:

```js
// The model._ioOptions property is a reference
// a shared object to store, so I need to clone
// it to add a per-request querystring.
model._ioOptions = Object.create(model._ioOptions);
model._ioOptions.query = "a=b&c=d";
```