[5.3] Fix serialization mistake on notifications

[spyric]

If we will use next notification:

<?php

namespace App\Notifications\Users;

use App\User;
use Illuminate\Bus\Queueable;
use Illuminate\Notifications\Notification;

class NewColleague extends Notification
{
    /**
     * @var
     */
    public $colleague;

    /**
     * Create a new notification instance.
     * @param User $colleague
     */
    public function __construct(User $colleague)
    {
        $this->colleague = $colleague;
    }

    /**
     * Get the notification's delivery channels.
     *
     * @param  mixed $notifiable
     * @return array
     */
    public function via($notifiable)
    {
        return [
            'broadcast',
            'database',
        ];
    }

    /**
     * Get the array representation of the notification.
     *
     * @param  mixed $notifiable
     * @return array
     */
    public function toArray($notifiable)
    {
        return [
            'text' => 'We have new colleague, his name is ' . $this->colleague->name,
            'icon' => 'fa fa-user-plus',
            'link' => route('employees.show', [$this->colleague]),
        ];
    }
}

It throw an Exception because when It execute toArray for database, $this->colleague is serialized. I think caused by Broadcast job creating.

If you swap database and broadcast all will be fine.

This patch fixes this issue