public function add($key, $value)
    {
        if (!array_key_exists($key, $this->data)) {
            $this->data[$key] = $value;
        } elseif (is_array($this->data[$key])) {
            $this->data[$key][] = $value;
        } else {
            $this->data[$key] = [$this->data[$key], $value];
        }

        return $this;
    }