def test_dunder_contains(self, d: HTTPHeaderDict) -> None:
        """
        Test:

        HTTPHeaderDict.__contains__ returns True
          - for matched string objects
          - for case-similar string objects
        HTTPHeaderDict.__contains__ returns False
          - for non-similar strings
          - for non-strings, even if they are keys
            in the underlying datastructure
        """
        assert "cookie" in d
        assert "CoOkIe" in d
        assert "Not a cookie" not in d

        marker = object()
        d._container[marker] = ["some", "strings"]  # type: ignore[index]
        assert marker not in d
        assert marker in d._container

        Test:

        HTTPHeaderDict.__contains__ returns True
          - for matched string objects
          - for case-similar string objects
        HTTPHeaderDict.__contains__ returns False
          - for non-similar strings
          - for non-strings, even if they are keys
            in the underlying datastructure
        
test/test_collections.py