public function toArray()
    {
        $message = str_array_filter([
            'from' => $this->from,
            'message_format' => $this->format,
            'color' => $this->color,
            'notify' => $this->notify,
            'message' => $this->content,
            'attach_to' => $this->attachTo,
        ]);

        if (! is_null($this->card)) {
            $message['card'] = $this->card->toArray();
        }

        return $message;
    }