NullPromotionHandler: return self from #apply

The existing promotion handlers are activated using either `#apply` or
`#activate`. The return value `#activate` is usually not read, and is
thus not really important, but that of `#apply` must be the handler
itself, so that e.g. the adjustments admin has access to any application
errors.

core/app/models/spree/null_promotion_handler.rb

@@ -12,7 +12,10 @@ def initialize(order)
     def activate
       @order
     end
-    alias_method :apply, :activate
+
+    def apply
+      self
+    end
 
     def can_apply?
       true

core/spec/models/spree/null_promotion_handler_spec.rb

@@ -17,8 +17,9 @@
   describe "#apply" do
     subject(:activate) { handler.apply }
 
-    it "returns the unchanged order" do
-      expect(activate).to eq(order)
+    it "returns the handler with the unchanged order" do
+      expect(activate).to eq(handler)
+      expect(activate.order).to eq(order)
     end
   end