public void add(String headerName, String value) {
        String normalName = HeaderName.normalize(Objects.requireNonNull(headerName, "headerName"));
        Objects.requireNonNull(value, "value");
        addNormal(headerName, normalName, value);
    }
@Test
    void add() {
        Headers headers = new Headers();
        headers.add("Via", "duct");
        headers.add("Cookie", "this=that");
        headers.add("Cookie", "frizzle=frazzle");

        headers.add("via", "con Dios");

        Truth.assertThat(headers.getAll("Via"))
                .containsExactly("duct", "con Dios")
                .inOrder();
    }