[HttpFoundation] Enforce UTF-8 header in JSON responses.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This makes sure JSON responses are always explitly sent as UTF-8.
The RFC says that JSON "SHOULD" be sent as UTF-8, so that makes sense.

For reference, see [this old Laravel ticket](https://github.com/laravel/laravel/pull/910).