docs: public Response setImage(String bucket, String srcSiteUrl, String host) throws QiniuException {
          String encodedSiteUrl = UrlSafeBase64.encodeToString(srcSiteUrl);
          String encodedHost = null;
          if (host != null && host.length() > 0) {
              encodedHost = UrlSafeBase64.encodeToString(host);
          }
          String path = String.format("/image/%s/from/%s", bucket, encodedSiteUrl);
          if (encodedHost != null) {
              path += String.format("/host/%s", encodedHost);
          }
          return pubPost(path);
      }