Route Forwarding: receive multiple arguments `res.io.respond(a,b,c,...)`

I love the idea of Route Forwarding but somehow encounter a problem. I prefer pass result or error to browser with node style `[err, result, ...]` which is really common convention

```js
    try {
        var data = doSomething();
        res.io.respond(null, data);
    } catch(err) {
        res.io.respond(err);
    }
```

but it doesnt work with Route-Forwarding because express's `res.json` expect additional param as status code, so `res.io.respond(null, data)` => `res.json(null, data)` just crash express.

writing sth. like `res.io.respond(500, err)` works with current express.io but really sounds strange, so I send this PR to change this behavior. it does break backward compatibility but I believe it worth that