Promotion order adjuster: update adjustment labels

If a promotion calculator implements a dynamic
`#{adjustable}_adjustment_label` method, we need to update the
adjustment label when it changes. This makes sure that is the case.

Author: mamhoff

promotions/app/models/solidus_promotions/order_adjuster/persist_discounted_order.rb

@@ -70,9 +70,9 @@ def update_adjustment(item, discount_item)
         adjustment ||= item.adjustments.new(
           source: discount_item.source,
           order_id: item.is_a?(Spree::Order) ? item.id : item.order_id,
-          label: discount_item.label
         )
         adjustment.amount = discount_item.amount
+        adjustment.label = discount_item.label
         adjustment
       end
     end

promotions/spec/models/solidus_promotions/order_adjuster_spec.rb

@@ -80,6 +80,15 @@
           expect(adjustable.current_discounts).to be_empty
         end
 
+        it "updates adjustment labels" do
+          subject
+          expect_any_instance_of(SolidusPromotions::Benefit).to receive(:adjustment_label).and_return("Changed Adjustment Label")
+          expect { order_adjuster.call }
+            .to change { order.line_items.first.adjustments.first.label }
+            .from("Promotion (Because we like you)")
+            .to("Changed Adjustment Label")
+        end
+
         context "if order is complete but not shipped" do
           let(:line_item) { order.line_items.first }
           let(:order) { create(:order_ready_to_ship) }