Always overwrite `Content-Type` for JSON responses

```js
route.get('/json', ({ response }) => {
  response.type = 'application/vnd.myapi.v1+json'
  return { message: 'hello world!' }
})
```
will always set the `Content-Type` to `application/json; charset=utf-8` instead of `application/vnd.myapi.v1+json`