Conditionally persist Shipment#update_amounts changes

This is in service of supporting the InMemoryOrderUpdater's goal to not do database writes.

core/app/models/spree/shipment.rb

@@ -258,10 +258,10 @@ def tracking_url
       @tracking_url ||= shipping_method.build_tracking_url(tracking)
     end
 
-    def update_amounts
+    def update_amounts(persist: true)
       if selected_shipping_rate
         self.cost = selected_shipping_rate.cost
-        if changed?
+        if changed? && persist
           update_columns(
             cost: cost,
             updated_at: Time.current

core/spec/models/spree/shipment_spec.rb

@@ -508,22 +508,41 @@
   end
 
   describe "#update_amounts" do
-    let(:shipment) { create(:shipment, cost: 3) }
+    subject { shipment.update_amounts(persist: persist) }
+
+    let(:persist) { true }
+    let(:shipment) { create(:shipment, cost: 1) }
 
     context 'when the selected shipping rate cost is different than the current shipment cost' do
-      before { shipment.selected_shipping_rate.update!(cost: 5) }
+      before { shipment.selected_shipping_rate.update!(cost: 999) }
 
-      it "updates the shipments cost" do
+      it "changes and persists the shipments cost" do
         expect {
-          shipment.update_amounts
-        }.to change { shipment.reload.cost }.to(5)
+          subject
+        }.to change { shipment.reload.cost }.to(999)
       end
 
-      it 'changes the updated_at column' do
+      it 'changes and persists the updated_at column' do
         expect {
-          shipment.update_amounts
+          subject
         }.to change { shipment.reload.updated_at }
       end
+
+      context 'when `persist: false` is passed' do
+        let(:persist) { false }
+
+        it 'does not perform any database writes' do
+          expect {
+            subject
+          }.not_to make_database_queries(manipulative: true)
+        end
+
+        it "changes but does not persist the shipments cost" do
+          subject
+          expect(shipment.cost).to eq 999
+          expect(shipment.reload.cost).to eq 1
+        end
+      end
     end
   end