[5.6] Add ability to extend DatabaseNotification model attributes on create

[antonkomarev]

Building of attributes array for the Illuminate\Notifications\DatabaseNotification model inside Illuminate\Notifications\Channels\DatabaseChannel extracted to method, what allow us to extend or overwrite default attributes.

I found it useful because I were needed to add seen_at and some more attributes related to my application and were needed to extend default DatabaseNotification model. And instead just modifying attributes in DatabaseChannel class on notification creation, we need to copy-paste send method. That means that if there will be changes in send method in the future - we will require to update extended implementations.

With this change we can do:

<?php

namespace App\Notification\Channels;

use Illuminate\Notifications\Channels\DatabaseChannel as BaseDatabaseChannel;
use Illuminate\Notifications\Notification;

class DatabaseChannel extends BaseDatabaseChannel
{
    protected function buildPayload($notifiable, Notification $notification)
    {
        return array_merge(parent::buildPayload($notifiable, $notification), [
            'seen_at' => null,
            'after_updrade' => 'Sleep well',
        ];
    }
}

Additionally it helps us to keep the code DRY.