Provide API for resetting order discounts on order and shipment

The order can be called to reset discounts on all leaf items. This is
what does it.

promotions/app/decorators/models/solidus_friendly_promotions/order_decorator.rb

@@ -20,6 +20,11 @@ def ensure_promotions_eligible
       super
     end
 
+    def reset_current_discounts
+      line_items.each(&:reset_current_discounts)
+      shipments.each(&:reset_current_discounts)
+    end
+
     Spree::Order.prepend self
   end
 end

promotions/app/decorators/models/solidus_friendly_promotions/shipment_decorator.rb

@@ -3,5 +3,12 @@
 module SolidusFriendlyPromotions
   module ShipmentDecorator
     Spree::Shipment.prepend SolidusFriendlyPromotions::DiscountableAmount
+
+    def reset_current_discounts
+      super
+      shipping_rates.each(&:reset_current_discounts)
+    end
+
+    Spree::Shipment.prepend self
   end
 end

promotions/spec/models/spree/order_spec.rb

@@ -5,4 +5,18 @@
 RSpec.describe Spree::Order do
   it { is_expected.to have_many :friendly_promotions }
   it { is_expected.to have_many :friendly_order_promotions }
+
+  describe "#reset_current_discounts" do
+    let(:line_item) { Spree::LineItem.new }
+    let(:shipment) { Spree::Shipment.new }
+    let(:order) { Spree::Order.new(shipments: [shipment], line_items: [line_item]) }
+
+    subject { order.reset_current_discounts }
+
+    it "resets the current discounts on all line items and shipments" do
+      expect(line_item).to receive(:reset_current_discounts)
+      expect(shipment).to receive(:reset_current_discounts)
+      subject
+    end
+  end
 end

promotions/spec/models/spree/shipment_spec.rb

@@ -20,5 +20,20 @@
 
       it { is_expected.to eq(18) }
     end
+
+    describe "#reset_current_discounts" do
+      let(:shipping_rate) { Spree::ShippingRate.new }
+      let(:shipment) { Spree::Shipment.new(shipping_rates: [shipping_rate]) }
+
+      subject { shipment.reset_current_discounts }
+      before do
+        shipment.current_discounts << SolidusFriendlyPromotions::ItemDiscount.new(item: double, amount: -2, label: "Foo", source: double)
+      end
+
+      it "resets the current discounts to an empty array and resets current discounts on all shipping rates" do
+        expect(shipping_rate).to receive(:reset_current_discounts)
+        expect { subject }.to change { shipment.current_discounts.length }.from(1).to(0)
+      end
+    end
   end
 end