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

[mcfog]

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

    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