Explicit-Null-Body should contains Content-Length
Koa version 2.13.1

Running below codes:
```js
const koa = require('koa');
const app = new koa();

app.use(ctx => {
  ctx.body = null;
  ctx.status = 404;
});

app.listen(3000, () => {} )
```

Both in chrome and curl test, it fails in 5s.

```shell
$> time curl http://localhost:3000/
curl http://localhost:3000/  0.01s user 0.01s system 0% cpu 5.013 total
```

I think the question is lost of Content-Length header:
> A user agent SHOULD send a Content-Length in a request message when
   no Transfer-Encoding is sent and the request method defines a meaning
   for an enclosed payload body. 

from [RFC7230](https://tools.ietf.org/html/rfc7230#section-3.3.2)

I add one line codes `ctx.length = 0;` in [application module](https://github.com/koajs/koa/blob/master/lib/application.js#L258), it pass all test cases and works for chrome and curl.

If it is really a bug, and the modification works, I will submit a PR.