Update class-wc-coupon.php

If you loaded custom coupon data using the  `'woocommerce_get_shop_coupon_data'` filter. Only `$coupon->amount` was set. While the 'normal' coupons loaded from the database get both `$coupon->coupon_amount` and `$coupon->amount` set.

This change also adds `$coupon->coupon_amount` for in code generated coupons and allows functions using the `'woocommerce_get_shop_coupon_data'` filter to use either `amount` or `coupon_amount`.