ImmutableList.Builder<String> httpHeaders = ImmutableList.builder();
    for (Field field : httpHeadersFields()) {
      httpHeaders.add((String) field.get(null));
    }
    assertThat(httpHeaders.build()).containsNoDuplicates();
  }

  private static ImmutableSet<Field> httpHeadersFields() {
    ImmutableSet.Builder<Field> builder = ImmutableSet.builder();