docs: public CdnResult.PrefetchResult prefetchUrls(String[] urls) throws QiniuException {
          if (urls != null && urls.length > MAX_API_PREFETCH_URL_COUNT) {
              throw new QiniuException(new Exception("url count exceeds the max prefetch limit per request"));
          }
          HashMap<String, String[]> req = new HashMap<>();
          req.put("urls", urls);
          byte[] body = Json.encode(req).getBytes(Constants.UTF_8);
          String url = server + "/v2/tune/prefetch";
          StringMap headers = auth.authorizationV2(url, "POST", body, Client.JsonMime);
          Response response = client.post(url, body, headers, Client.JsonMime);
          return response.jsonToObject(CdnResult.PrefetchResult.class);
      }