Add getter for configured class names

When configuring Flickwerk, we need a way to get at class names without
constantizing (i.e. loading) those classes. This adds some specs for the
`class_name_attribute` class method on
Spree::Preferences::Configuration, and generates a new getter that has
`_name` attached to the method to get just the string name.

core/lib/spree/preferences/configuration.rb

@@ -173,6 +173,10 @@ def self.class_name_attribute(name, default:)
         class_name = class_name.constantize if class_name.is_a?(String)
         class_name
       end
+
+      define_method("#{name}_name") do
+        instance_variable_get(ivar) || default
+      end
     end
 
     def self.by_version(*args)

core/spec/models/spree/preferences/configuration_spec.rb

@@ -7,6 +7,7 @@
     Class.new(Spree::Preferences::Configuration) do
       preference :color, :string, default: :blue
       versioned_preference :foo, :boolean, initial_value: true, boundaries: { "3.0" => false }
+      class_name_attribute :order_recalculator_class, default: 'Spree::OrderUpdater'
     end.new
   end
 
@@ -84,4 +85,16 @@
       end
     end
   end
+
+  describe ".class_name_attribute" do
+    it "allows getting the constant of a configurable class" do
+      config.order_recalculator_class = 'Spree::OrderUpdater'
+      expect(config.order_recalculator_class).to eq Spree::OrderUpdater
+    end
+
+    it "allows getting the string name of the class" do
+      config.order_recalculator_class = 'Spree::OrderUpdater'
+      expect(config.order_recalculator_class_name).to eq 'Spree::OrderUpdater'
+    end
+  end
 end