Configuration overrides are not applied due to use of editable configuration for getting headers

@holo96: Thank, using ImmutableConfig for getting definitely always makes sense!
Could you create a MR for fixing this?

Then I think we'll be able to get this resolved fast!

It's a one-line fix, but I added Kernel tests around it to verify.

Note, if you wish to override to a completely empty array, you need to use the following syntax in settings.php (see https://www.drupal.org/project/drupal/issues/2990549 🐛 Impossible to config override a key's value to an empty array Needs work )

$config['remove_http_headers.settings']['headers_to_remove'] = new \EmptyIterator();

The MR shows tests failing, but it's not the tests added by this MR (which are passing), so leaving at Needs Review.

Thanks, could you maybe create a separate issue for that?