When your Value Object is so immutable, you remove a field and a non-deterministic value is readded in its place

gistfile1.txt

<?php
require 'vendor/autoload.php';
$response = Symfony\Component\HttpFoundation\Response::create('HelloWorld');
$response->headers->remove('Date'); 
$response->headers->remove('date'); // just to be sure
var_dump($response->headers->all());
/*
array(2) {
  ["cache-control"]=>
  array(1) {
    [0]=>
    string(17) "no-cache, private"
  }
  ["date"]=>
  array(1) {
    [0]=>
    string(29) "Tue, 23 Jan 2018 11:53:09 GMT"
  }
}
*/
// see https://github.com/symfony/http-foundation/blob/master/ResponseHeaderBag.php#L39-L42