@Override
    public Headers add(Header header) {
        Objects.requireNonNull(header, "Unable to add a null header.");
        if (headers == null) {
            headers = new LinkedList<>();
        }
        headers.add(header);
        return this;
    }
@Test
    public void shouldNotAllowToAddNullHeader() {
        final ConnectHeaders headers = new ConnectHeaders();
        assertThrows(NullPointerException.class, () -> headers.add(null));
    }