Add a spec for #transfer_to_location to verify the attributes of the new

shipment after the split.

core/spec/models/spree/shipment_spec.rb

@@ -24,6 +24,26 @@
   let(:variant) { mock_model(Spree::Variant) }
   let(:line_item) { mock_model(Spree::LineItem, variant: variant) }
 
+  context '#transfer_to_location' do
+    it 'transfers unit to a new shipment with given location' do
+      order = create(:completed_order_with_totals, line_items_count: 2)
+      shipment = order.shipments.first
+      variant = order.inventory_units.map(&:variant).first
+
+      aggregate_failures("verifying new shipment attributes") do
+        expect do
+          shipment.transfer_to_location(variant, 1, stock_location)
+        end.to change { Spree::Shipment.count }.by(1)
+
+        new_shipment = order.shipments.last
+        expect(new_shipment.number).to_not eq(shipment.number)
+        expect(new_shipment.stock_location).to eq(stock_location)
+        expect(new_shipment.line_items.count).to eq(1)
+        expect(new_shipment.line_items.first.variant).to eq(variant)
+      end
+    end
+  end
+
   # Regression test for https://github.com/spree/spree/issues/4063
   context "number generation" do
     before { allow(order).to receive :update! }