Limit usage not updating

Hi ALberto,
thanks for developing this plugin.

When I configure the coupon in single product page, there are some meta_keys that are not being updated. For example: product_ids, usage_limit, usage_limit_per_user and limit_usage_to_x_items.

My version of wordpress is 4.8.1 and woocommerce 3.1.2

Thank you for your help.