Some attribute in ctx.response can not printed by console.log
```
const Koa = require('koa');
const app = new Koa();

app.use(async ctx => {
  ctx.body = 'Hello World';
  console.log(ctx.response);
});

app.listen(3000);
```

I just want to see the attributions in ctx.response, but just status | message | header | body are shown.

It should have some other attributes like type | length | etag or app | res | req ...

Thx!