code: public static String printToUnicodeString(final UnknownFieldSet fields) {
      try {
        final StringBuilder text = new StringBuilder();
        UNICODE_PRINTER.printUnknownFields(fields, new TextGenerator(text));
        return text.toString();
      } catch (IOException e) {
        throw new IllegalStateException(e);
      }
    }
docs: Same as {@code printToString()}, except that non-ASCII characters
  in string type fields are not escaped in backslash+octals.