Replacing superclass_delegating_accessoror with class_attribute.

:store_full_sti_class will have value true or false so there is no
danger of mutability here.

Signed-off-by: José Valim <jose.valim@gmail.com>

Author: Neeraj Singh

activerecord/lib/active_record/base.rb

@@ -421,7 +421,7 @@ def subclasses
     @@timestamped_migrations = true
 
     # Determine whether to store the full constant name including namespace when using STI
-    superclass_delegating_accessor :store_full_sti_class
+    class_attribute :store_full_sti_class
     self.store_full_sti_class = true
 
     # Stores the default scope for the class