Support Map object in response body
Currently, koa.js does not support JavaScript Map Object in the response body.

Now, if a Map object is assigned to it, the front-end receives nothing but an empty object, as shown in [this repository](https://codesandbox.io/s/investigation-koa-router-map-gzz6n?file=/src/index.js).

The current work-around is calling `Object.fromEntries()` to convert a map to an object, then send the object to the front-end. It would be better to support this directly.

Related: koajs/router#120