Rest API: Ensure `rest_ensure_response()` upgrades `WP_HTTP_Response` to `WP_REST_Response`.

An instance of `WP_HTTP_Response` doesn't ensure that the required methods used in `WP_REST_Server::dispatch()` exist, currently causing a fatal error.

Props ali11007, TimothyBlynJacobs, ocean90.
Fixes #49495.
Built from https://develop.svn.wordpress.org/trunk@47849


git-svn-id: https://core.svn.wordpress.org/trunk@47625 1a063a9b-81f0-0310-95a4-ce76da25c4cd