Add category filtering feature

Hello,
I modified the plugin in order to accept category filtering and replaced an array_merge with a simple cast as array in function

public function mod_woocommerce_product_query($q, $WC_Query){
		
		global $wp_query;

		if($wp_query->is_sale_page) {

			$product_ids_on_sale = apply_filters( 'wc_onsale_page_product_ids_on_sale', wc_get_product_ids_on_sale() );
			$meta_query = WC()->query->get_meta_query();
			$q->set( 'post__in', (array) $product_ids_on_sale );
			if (isset($_GET['product-category']))
				$q->set( 'product_cat', $_GET['product-category'] );
		}
	}

This could be a good addition if someone wants to filter out certain categories dynamically which are on sale.