Only manipulate headers if not already sent

[BlackbitDevs]

When you want to send headers before request finishes via

class MyController  {
  public function myAction() {
      $response = new Response();
      $response->headers->set(
          'Content-Disposition',
          HeaderUtils::makeDisposition(
              HeaderUtils::DISPOSITION_ATTACHMENT,
              'file.txt
          )
      );
      
      $response->sendHeaders();
      ob_flush();
      flush();

      // do some work to populate response
      sleep(5); // this is only for demonstraton purposes

      return $response;
  }
}

I get a warning error

E_WARNING: Cannot modify header information - headers already sent in [Root folder]/vendor/symfony/http-foundation/Session/SessionUtils.php, line 54

With this PR the headers get only manipulated during kernel.response event if the headers have ot already been sent.

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony repository. It is used by Composer to allow developers to depend on specific Symfony components.

If you want to contribute, you should instead open a pull request on the main repository:

https://github.com/symfony/symfony

Thank you for your contribution!

PS: if you haven't already, please add tests, and beware that bug fixes should be submitted on the lowest maintained branch where they apply.

[BlackbitDevs]

Closed in favor of symfony/symfony#45434