Fix migration error

[faenir]

create table shipping_discount ... done (time: 0.078s) Exception 'yii\base\UnknownPropertyException' with message 'Getting unknown property: app\modules\shop\models\DiscountType::custom_view'

custom_view field added by m170408_060716_disctype_add_customview.php migration (after m151103_105905_shipping_discount). DiscountType model has validation rule for custom_view field. m151103_105905_shipping_discount -> $discountType->save() - runs validation for not exists field.

[bethrezen]

Thanks!