WooCommerce Account Funds 3.1.1

* Tweak - Remove the discount applied by the "Account funds" payment gateway when this one stops being available.
* Tweak - Remove the applied funds if after updating the cart the "Account funds" payment gateway is available.
* Tweak - Set the minimum Top-up amount to 1.
* Tweak - Tested compatibility up to WC 5.3.
* Fix - Don't add to cart a Top-up product with zero funds.

* Fix - Percentage discount for using funds is calculated based on the amount of funds used (Reverts the fix 2.4.1).

* Fix - Fixed wrong discount when giving a percentage discount for using funds.