docs: public String saveAs(String streamKey, String fileName, long start, long end, StringMap other)
              throws QiniuException {
          String path = encodeKey(streamKey) + "/saveas";
          StringMap param = other != null ? other : new StringMap();
          param.putNotEmpty("fname", fileName).put("start", start).put("end", end);
          String body = param.jsonString();
          SaveRet r = post(path, body, SaveRet.class);
          return r.fname;
      }