code: String dumpAsString() throws IOException {
          StringBuilder sb = new StringBuilder();
          OutputStream ops = new StringOutputStream(sb, Encoding.defaultEncoding());
          dump(ops);
          return sb.toString();
      }
docs: 将标记的内容 Dump 成一个字符串,使用默认字符集
  
  @return 字符串
  
  @throws IOException