add GetRefund (#286)

pay/pay.go

@@ -4,6 +4,7 @@ import (
 	"github.com/silenceper/wechat/v2/pay/config"
 	"github.com/silenceper/wechat/v2/pay/notify"
 	"github.com/silenceper/wechat/v2/pay/order"
+	"github.com/silenceper/wechat/v2/pay/refund"
 )
 
 //Pay 微信支付相关API
@@ -25,3 +26,8 @@ func (pay *Pay) GetOrder() *order.Order {
 func (pay *Pay) GetNotify() *notify.Notify {
 	return notify.NewNotify(pay.cfg)
 }
+
+// GetRefund  退款
+func (pay *Pay) GetRefund() *refund.Refund {
+	return refund.NewRefund(pay.cfg)
+}