Issue #3214208 by gapple, Akhil Babu, larowlan, smustgrave:...

    $finishSubscriber->onRespond($event);

    $this->assertEquals(['en'], $response->headers->all('Content-language'));

    // 'X-Content-Type-Options' will be unconditionally set by the core.

    // 'X-Content-Type-Options' will be unconditionally set by core.

    $this->assertEquals(['nosniff'], $response->headers->all('X-Content-Type-Options'));

    $this->assertEquals(['DENY'], $response->headers->all('X-Frame-Options'));

  }