check if AsPivot trait is used instead of Pivot Model

[Gummibeer]

Right now the logic that allows using a class name of a pivot model as $table argument and resolves its table and also uses it as pivot model only checks for the default \Illuminate\Database\Eloquent\Relations\Pivot model class.

The problem with this is that the real thing that makes a pivot model is the \Illuminate\Database\Eloquent\Relations\Concerns\AsPivot trait.

This PR changes this condition to check for the trait instead of an instance check for the base class.

The benefit is that the user is able to create his own pivot base model that doesn't have to be based on the default pivot model.

As an example - that's our custom pivot model in one app that's based on our custom base model.

namespace App\Eloquent;

use Illuminate\Database\Eloquent\Relations\Concerns\AsPivot;
use Illuminate\Support\Str;

abstract class Pivot extends Model
{
    use AsPivot;

    public $incrementing = true;

    public function getTable(): string
    {
        return $this->table ?? Str::snake(Str::singular(Str::replaceLast('Pivot', '', class_basename($this))));
    }
}