Expose original spark stream as _spark.

I use this to identify requests in sharejs middleware like this, as I haven't found another way to match request objects in sharejs to their underlying connection.

```js
function submitMiddleware(request, next) {
  var id = request.agent.session.stream._spark.id;
  if (!auth(id)) {
    return next('Not authenticated');
  } 
  next();
}
```