improvements on discount calculations

lib/src/discount.dart

@@ -22,10 +22,15 @@ class Discount {
   })  : assert(amount >= 0, 'Amount must be zero or positive.'),
         amount = !isPercentage || amount <= 1 ? amount : amount / 100;
 
+  bool get affectsTotal => !affectSubtotalBeforeTaxes && amount > 0;
+
   static Discount empty() => Discount(amount: .0);
 
   /// Calculates discount amount for given [value].
   double discountOf(double value) {
+    if (amount == 0) {
+      return 0;
+    }
     if (isPercentage) {
       return value * amount;
     }

lib/src/item.dart

@@ -36,7 +36,9 @@ class Item implements Sellable {
   /// Calculates subtotal amount including [discount] (before taxes).
   @override
   double get subtotal {
-    if (discount == null || discount!.affectSubtotalBeforeTaxes) {
+    if (discount == null ||
+        discount!.affectSubtotalBeforeTaxes ||
+        discount!.amount == 0) {
       return _subtotalOf(unitPrice, discount);
     }
     final base = _subtotalOf(unitPrice);

lib/src/sale.dart

@@ -84,7 +84,7 @@ class Sale implements Sellable {
   }
 
   Discount? get _discount {
-    if (discount != null && !discount!.affectSubtotalBeforeTaxes) {
+    if (discount?.affectsTotal ?? false) {
       final baseSubtotal =
           items.map((item) => item.subtotal).reduce((s1, s2) => s1 + s2);
       final baseTax = items.map((item) => item.tax).reduce((s1, s2) => s1 + s2);