Fix specs that fail under Rails 5.2.1

I'm not sure why but previously the association was returning an
instance of ActiveRecord::Associations::CollectionProxy that was
considered not changed while reloading. Despite that `shipments`
call is still returning the same object, they now have a different
`object_id` and this could be the root of the problem.

I'm not sure if this issue raised up with Rails 5.2.1 or by using the
ransack master branch on github.

core/spec/models/spree/order_spec.rb

@@ -379,13 +379,13 @@ def merge!(other_order, user = nil)
 
         it "does nothing if any shipments are ready" do
           shipment = create(:shipment, order: subject, state: "ready")
-          expect { subject.ensure_updated_shipments }.not_to change { subject.reload.shipments }
+          expect { subject.ensure_updated_shipments }.not_to change { subject.reload.shipments.pluck(:id) }
           expect { shipment.reload }.not_to raise_error
         end
 
         it "does nothing if any shipments are shipped" do
           shipment = create(:shipment, order: subject, state: "shipped")
-          expect { subject.ensure_updated_shipments }.not_to change { subject.reload.shipments }
+          expect { subject.ensure_updated_shipments }.not_to change { subject.reload.shipments.pluck(:id) }
           expect { shipment.reload }.not_to raise_error
         end
       end
@@ -1102,11 +1102,12 @@ def generate
 
     it "raises an error if any shipments are ready" do
       shipment = create(:shipment, order: subject, state: "ready")
+
       expect {
         expect {
           subject.create_proposed_shipments
         }.to raise_error(Spree::Order::CannotRebuildShipments)
-      }.not_to change { subject.reload.shipments }
+      }.not_to change { subject.reload.shipments.pluck(:id) }
 
       expect { shipment.reload }.not_to raise_error
     end
@@ -1117,7 +1118,7 @@ def generate
         expect {
           subject.create_proposed_shipments
         }.to raise_error(Spree::Order::CannotRebuildShipments)
-      }.not_to change { subject.reload.shipments }
+      }.not_to change { subject.reload.shipments.pluck(:id) }
 
       expect { shipment.reload }.not_to raise_error
     end