koa v2: ctx.status initially 404 rather than undefined.
The  [koa-v2 documentation](https://github.com/koajs/koa/blob/v2.x/docs/api/response.md#responsestatus) states that:

> By default, response.status is not set unlike node's res.statusCode which defaults to 200.

Yet, it appears to be initially set to 404. If I make a request to the following simple test server:

```js
const app = new Koa();
app.use(ctx => console.log('status: ' + ctx.status));   // or ctx.response.status
const server = app.listen();
```

I get `status: 404` on the console.

