docs: private RegisterDocumentResponse registerDocument(RegisterDocumentRequest request) {
          checkNotNull(request, "The parameter request should NOT be null.");
  
          InternalRequest internalRequest = createRequest(HttpMethodName.POST, request, DOC);
          internalRequest.addParameter("register", null);
          String strJson = JsonUtils.toJsonString(request);
          byte[] requestJson = null;
          try {
              requestJson = strJson.getBytes(DEFAULT_ENCODING);
          } catch (UnsupportedEncodingException e) {
              throw new BceClientException("Unsupported encode.", e);
          }
  
          internalRequest.addHeader(Headers.CONTENT_LENGTH, String.valueOf(requestJson.length));
          internalRequest.addHeader(Headers.CONTENT_TYPE, DEFAULT_CONTENT_TYPE);
          internalRequest.setContent(RestartableInputStream.wrap(requestJson));
          return invokeHttpClient(internalRequest, RegisterDocumentResponse.class);
      }