code: private String downloadHeadImg(String userName) {
          return this.downloadFile(
                  new DownLoad(ApiURL.HEAD_IMG, bot.session().getUrl(), userName, bot.session().getSKey())
                          .msgId(userName)
          );
      }
docs: 下载头像到本地
  
  @param userName 用户名
  @return 返回头像本地路径