[8.x] Fix fail to morph with custom casting to objects

[marandy0905]

While Laravel introduce custom cast in 7.x, it's not really that compatible with morph map.

For a morphTo relation to work, it relies on accessing the actual data with the __get() method of the model to assume whether we're or not eager loading the relationship. See code here.

By using the __get() method of the model, we'll be applying the cast to the attribute we're asking for. This works great before custom cast are introduced.

However if a custom cast is applied on the attribute, the attribute might be casted to any object, which is incompatible with Relation::morphMap() (returning an array with the key as int/string).

Here's an example setup:

class Logs extends Model
{
    protected $table = 'logs';

    protected $fillable = ['role_type', 'role_user_id'];

    protected $casts = [
        'role' => CastToRoleTypeEnum::class,
    ];

    public function roleUser()
    {
        return $this->morphTo(__FUNCTION__, 'role_type', 'role_user_id', 'id');
    }
}

class Admins extends Model
{
    protected $table = 'admins';
}

class Users extends Model
{
    protected $table = 'users';
}

class CastToRoleTypeEnum implements CastsAttributes
{
    public function get($model, $key, $value, $attributes)
    {
        return new RoleTypeEnum($value);
    }

    public function set($model, $key, $value, $attributes)
    {
        return $value->getValue();
    }
}

class RoleTypeEnum
{
    public function __construct($value)
    {
        $this->value = $value;
    }

    public function getValue()
    {
        return $this->value;
    }
}

// In AppServiceProvider.php, define the morph map
// Also remember to define Admin/User model which I will not demonstrate here to save space
Relation::morphMap([
    'admin' => 'App\Models\Admins',
    'user' => 'App\Models\Users',
]);

The morphMap above uses the keys of an array as the custom polymorphic types to map to the correspond models, which is impossible to map while the magic __get method in Model.php will be always casting it to a LogRole object in morphTo.

Sorry this is my first pull request to Laravel, I've tried my best to understand the contribution guide and explain my problem, please feel free to ask any question and I'll try my best to reply as soon as possible.