Let Order#all_inventory_units_returned? always use fresh data

This method is mainly used after inventory items are returned, and
that usually happens through customer_returns -> return_item ->
inventory_unit making the instances of InventoryUnit that are local to
the order stale. By using reload we avoid the bugs stemming from this
discrepancy.

core/app/models/spree/order.rb

@@ -288,7 +288,12 @@ def allow_cancel?
     end
 
     def all_inventory_units_returned?
-      inventory_units.all?(&:returned?)
+      # Inventory units are transitioned to the return state by ReturnItem
+      # across a different association path goes through CustomerReturn and
+      # ReturnItem instead of using Order#inventory_units. That makes any
+      # the cached association on order stale and require a *reload* before
+      # checking if they're returned.
+      inventory_units.reload.all?(&:returned?)
     end
 
     def contents

core/spec/models/spree/order_spec.rb

@@ -1187,6 +1187,18 @@ def generate
         expect(subject).to eq false
       end
     end
+
+    context "all inventory units are returned on the database (e.g. through another association)" do
+      it "is true" do
+        expect {
+          Spree::InventoryUnit
+            .where(id: order.inventory_unit_ids)
+            .update_all(state: 'returned')
+        }.to change {
+          order.all_inventory_units_returned?
+        }.from(false).to(true)
+      end
+    end
   end
 
   context "store credit" do