[11.x] Add withoutHeader() test method

[gdebrauwer]

You can add a header using the withHeader() method in a test.

$this->withHeader('Foo', 'bar')->get(...);

There is currently no method to remove a header. This PR fixes that:

$this->withoutHeader('Foo')->get(...);

This can be useful if you set a header in the setUp() method of the base TestCase class so the header is present on all requests in the test suite, but you want to remove that header in one or more specific tests in that entire test suite.

I also updated the existing withoutToken() method to use the withoutHeader() method in the background.