public function add($key, $value)
    {
        if (! is_null($key)) {
            $currentValue = Arr::get($this->payload, $key, []);

            if (! is_array($currentValue)) {
                $currentValue = Arr::wrap($currentValue);
            }

            $currentValue[] = $value;

            Arr::set($this->payload, $key, $currentValue);
        }

        return $this;
    }