docs: public String pfop(String bucket, String key, String fops, StringMap params) throws QiniuException {
          params = params == null ? new StringMap() : params;
          params.put("bucket", bucket).put("key", key).put("fops", fops);
          byte[] data = StringUtils.utf8Bytes(params.formString());
          String url = configuration.apiHost(auth.accessKey, bucket) + "/pfop/";
          StringMap headers = auth.authorization(url, data, Client.FormMime);
          Response response = client.post(url, data, headers, Client.FormMime);
          if (!response.isOK()) {
              throw new QiniuException(response);
          }
          PfopResult status = response.jsonToObject(PfopResult.class);
          response.close();
          if (status != null) {
              return status.persistentId;
          }
          return null;
      }