By default, when a URL fail to be downloaded, the URL is blacklisted so the library won't keep trying.
This flag disable this blacklisting.默认情况下, 当一个 图片URL下载失败, 这个 URL 会被加入黑名单, 所以 SDWebImage 不会再下载这个资源.
SDWebImageRetryFailed 这个标记可以取消黑名单.
SDWebImageRetryFailed = 1 << 0,
[cell.imageView setImageWithURL:urlt
placeholderImage:[UIImage imageNamed:@"default.jpg"]
options:SDWebImageRetryFailed];
