public String privateDownloadUrl(String baseUrl, long expires) {
        long deadline = System.currentTimeMillis() / 1000 + expires;
        return privateDownloadUrlWithDeadline(baseUrl, deadline);
    }
public String privateDownloadUrl ( String baseUrl , long expires ) { long deadline = System . currentTimeMillis ( ) / 1000 + expires ; return privateDownloadUrlWithDeadline ( baseUrl , deadline ) ; }
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