WooCommerce Smart Coupons 8.22.0

* New: Allow use of store credit along with coupons which are Individual use coupons
* Update: Added more parameters in call to wc_mail, to allow custom headers & attachments
* Fix: Scheduled coupons won't display in the cart when published
* Fix: Unnecessarily modifying recurring total when store credit is applied to cart which contains subscription
* Fix: Order totals wrong when recalculation happens on order
* Fix: Error on product object (Call to member function on boolean)
* Fix: Bulk generation ignores excluded products
* Fix: Unnecessarily loading list of coupons, resulting in additional load on page
* Fix: Incorrect display of store credit amount in the order complete email when product is both virtual and downloadable
* Fix: No coupons sent on purchasing a variable subscription
* Fix: Illegal string offset
* Update: POT file

* Fix: Coupons not auto generating after purchase if order paid via coupon
* Fix: Uncaught Error: Call to a member function get_used_coupons on boolean (compatibility with WooCommerce Subscriptions)
* Fix: Conflict with Store Credit: Unable to use coupons if "delete after use" is enabled
* Update: Support for discount types added by 3rd party plugins for auto-generation
* Tweak: Turn sub-classes into Singleton classes (Thanks to: Matt Harrison)
* Update: POT file

* Fix: Broken meta fields for coupon on Bulk Generate Coupon page (WooCommerce 3.1+)
* Fix: "Exclude sale items" restriction isn't being passed on to the generated coupon
* Update: POT file