code: function onSend(error, bytes) {
      completed += 1;
      if (calledback) {
        return;
      }
  
      if (error) {
        if (typeof callback === 'function') {
          calledback = true;
          callback(error);
        } else if (self.errorHandler) {
          calledback = true;
          self.errorHandler(error);
        }
        return;
      }
  
      if (bytes) {
        sentBytes += bytes;
      }
  
      if (completed === stat.length && typeof callback === 'function') {
        callback(null, sentBytes);
      }
    }
docs: Gets called once for each callback, when all callbacks return we will
  call back from the function
  @private