how to using koa-graphql with koa-router@7.0.0
i used koa-router and koa-graphql with koa2 like this:

``` js
var router = new Router();
router.use('/graphql',graphqlHTTP({schema: schema}));
app.use(router.routes());
```

when i visite `/graphql`. but the router not work.
how do i do. thx.
