code: protected void send(final String requestBody, final ResponseListener listener) {
          String contentType = headers.get(CONTENT_TYPE);
  
          if (contentType == null) {
              contentType = TEXT_PLAIN_CONTENT_TYPE;
          }
  
          // If the request body is an empty string, it should be treated as null
          RequestBody body = null;
          if (requestBody != null && requestBody.length() > 0) {
              body = RequestBody.create(MediaType.parse(contentType), requestBody);
          }
  
          sendRequest(null, listener, body);
      }
docs: Send this resource request asynchronously, with the given string as the request body.
  If the Content-Type header was not previously set, this method will set it to "text/plain".
  
  @param requestBody   The text to put in the request body
  @param listener      The listener whose onSuccess or onFailure methods will be called when this request finishes