public function addAttribute($attribute, $value)
    {
        $value = is_array($value) ? $value : [$value];

        if (!isset($this->attributes[$attribute])) {
            $this->attributes[$attribute] = [];
        }
        foreach ($value as $attrValue) {
            $this->attributes[$attribute][] = $attrValue;
        }

        return $this;
    }