public function test_rest_pre_serve_request_headers() {
    $post = $this->factory()->post->create_and_get(
      array(
        'post_title' => 'Hello World',
      )
    );

    $request = new WP_REST_Request( 'GET', '/oembed/1.0/embed' );
    $request->set_param( 'url', get_permalink( $post->ID ) );
    $request->set_param( 'format', 'xml' );

    $server   = new WP_REST_Server();
    $response = $server->dispatch( $request );
    $output   = get_echo( '_oembed_rest_pre_serve_request', array( true, $response, $request, $server ) );

    $this->assertNotEmpty( $output );

    $headers = xdebug_get_headers();

    $this->assertContains( 'Content-Type: text/xml; charset=' . get_option( 'blog_charset' ), $headers );
  }