@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 shouldNotAllowNullKey() {
        assertThrows(NullPointerException.class,
            () -> headers.add(null, "value", Schema.STRING_SCHEMA));
    }