code: public static ImmutableListMultimap.Builder<String, String> headerMapBuilder() {
          return ImmutableListMultimap.<String, String> builder()
                  .orderKeysBy(String.CASE_INSENSITIVE_ORDER);
      }
docs: Creates a special {@link ImmutableMap} builder that creates
  case-insensitive maps, as required for headers.
  
  @return a header map builder