Alias NullPromotionHandler#activate to #apply

The four promotion handlers in core have two different methods to set
them into motion. Cart, Shipping, and Page use #activate, but Coupon
uses #apply. We only need one null promotion handler, and since we just
want it to return the unchanged promotion, we just alias.

core/app/models/spree/null_promotion_handler.rb

@@ -12,6 +12,7 @@ def initialize(order)
     def activate
       @order
     end
+    alias_method :apply, :activate
 
     def can_apply?
       true

core/spec/models/spree/null_promotion_handler_spec.rb

@@ -14,6 +14,14 @@
     end
   end
 
+  describe "#apply" do
+    subject(:activate) { handler.apply }
+
+    it "returns the unchanged order" do
+      expect(activate).to eq(order)
+    end
+  end
+
   describe "#can_apply?" do
     subject { handler.can_apply? }
     let(:order) { double(Spree::Order, coupon_code: nil) }