docs: public Response copy(String fromBucket, String fromFileKey, String toBucket, String toFileKey, boolean force)
              throws QiniuException {
          String from = encodedEntry(fromBucket, fromFileKey);
          String to = encodedEntry(toBucket, toFileKey);
          String path = String.format("/copy/%s/%s/force/%s", from, to, force);
          return rsPost(fromBucket, path, null);
      }