fix: DownloadableItem "product" field type changed to ProductUnion (#833)

Author: kidunot89

includes/type/object/class-downloadable-item-type.php

@@ -76,7 +76,7 @@ public static function register() {
 						},
 					],
 					'product'            => [
-						'type'        => 'Product',
+						'type'        => 'ProductUnion',
 						'description' => __( 'Product of downloadable item.', 'wp-graphql-woocommerce' ),
 						'resolve'     => static function ( $source, array $args, AppContext $context ) {
 							return Factory::resolve_crud_object( $source['product_id'], $context );