Koa2 and Controller Actions -> this is undefined
in this example is `this` undefined

``` JavaScript
class Root {
    async index (ctx) {
        console.log(this) // <-- is undefined
        ctx.body "Root"
    }
}

const app = new Koa()
const root = new Root()

app.use(root.index)
app.listen(3000)
```

i have to explicity do this

``` JavaScript
// ...
app.use(root.index.bind(root))
// ...
```

That is uncool xD
