code: public String privateDownloadUrl(String baseUrl, long expires) {
          long deadline = System.currentTimeMillis() / 1000 + expires;
          return privateDownloadUrlWithDeadline(baseUrl, deadline);
      }
docs: 下载签名
  
  @param baseUrl 待签名文件url，如 http://img.domain.com/u/3.jpg 、
  http://img.domain.com/u/3.jpg?imageView2/1/w/120
  @param expires 有效时长，单位秒。默认3600s
  @return