Use default option for class_variable

There is a default option that pre-initialized the value.

I noticed it is used in the attributes api, in which our code is based.

Author: kbrock

lib/active_record/virtual_attributes.rb

@@ -38,8 +38,7 @@ def type
     ActiveRecord::Type.register(:symbol, Type::Symbol)
 
     included do
-      class_attribute :virtual_attributes_to_define, :instance_accessor => false
-      self.virtual_attributes_to_define = {}
+      class_attribute :virtual_attributes_to_define, :instance_accessor => false, :default => {}
     end
 
     module ClassMethods

lib/active_record/virtual_attributes/virtual_delegates.rb

@@ -10,8 +10,7 @@ module VirtualDelegates
       extend ActiveSupport::Concern
 
       included do
-        class_attribute :virtual_delegates_to_define, :instance_accessor => false
-        self.virtual_delegates_to_define = {}
+        class_attribute :virtual_delegates_to_define, :instance_accessor => false, :default => {}
       end
 
       module ClassMethods