docs: public String[] buckets() throws QiniuException {
          String url = String.format("%s/buckets", configuration.rsHost());
          Response res = get(url);
          if (!res.isOK()) {
              throw new QiniuException(res);
          }
          String[] buckets = res.jsonToObject(String[].class);
          res.close();
          return buckets;
      }