docs: public FetchRet fetch(String url, String bucket, String key) throws QiniuException {
          String resource = UrlSafeBase64.encodeToString(url);
          String to = encodedEntry(bucket, key);
          String path = String.format("/fetch/%s/to/%s", resource, to);
          Response res = ioPost(bucket, path);
          if (!res.isOK()) {
              throw new QiniuException(res);
          }
          FetchRet fetchRet = res.jsonToObject(FetchRet.class);
          res.close();
          return fetchRet;
      }