code: public String html() {
          if (html != null) {
              return html;
          }
  
          if (content == null) {
              return null;
          }
          if (charset == null) {
              charset = CharsetDetector.guessEncoding(content());
          }
          try {
              html = new String(content, charset);
          } catch (UnsupportedEncodingException e) {
              LOG.info("Exception when decoding "+ key(),e);
              return null;
          }
          return html;
      }
docs: 返回网页的源码字符串
  
  @return 网页的源码字符串