docs: public final String toJsonString(
      final String source) throws Exception {
      StringBuffer sb = new StringBuffer();
      boolean isStartSpaces = false;
      for (int i = 0; i < source.length(); i++) {
        char ch = source.charAt(i);
        isStartSpaces = addJsonChar(ch, sb, isStartSpaces);
      }
      return sb.toString();
    }