@Test
void appliesToEachHeader() {

    final HeaderFilter unit = eachHeader((k, v) -> v.toUpperCase());
    final HttpHeaders actual = unit.filter(
            HttpHeaders.empty()
                    .update("hello", "world")
                    .update("chao", "bambina")
                    .update("already", "UPPERCASE"));
    final HttpHeaders expected = HttpHeaders.empty()
            .update("hello", "WORLD")
            .update("chao", "BAMBINA")
            .update("already", "UPPERCASE");
    assertEquals(expected, actual);
}