Extending default routes don't work
``` js
router.get('/', async function(ctx, next) {
    ctx.state = {
        title: 'koa2 title'
    };

    await ctx.render('index', {});
})

router.get('/test', async function(ctx, next) {
    let data = {a:1, b:2}
    ctx.status = 200;
    ctx.body = data;
    await next();
});

// -> /test - 404
```

Do I made any mistakes? Thanks.
