Use of built-in URL object conflicts with supported Koa versions
Built-in [URL](https://nodejs.org/api/url.html#url_class_url), as used in [just one place in `response`](https://github.com/koajs/koa/blob/4f968298f97394e488297ec32c8e927a3a322076/lib/response.js#L292) was added in Node 6.13, while Koa `engines` is `"^4.8.4 || ^6.10.1 || ^7.10.1 || >= 8.1.4"`.
Should that be fixed?
Which way:
1. Introducing [whatwg-url](https://www.npmjs.com/package/whatwg-url)? 
2. Bumping `engines` a little?
3. Simulating `URL` object by adding hostname properties, etc?
4. Just document that?
5. Do nothing, nobody is using Node < 6.13 today.