Remove method if already exists to avoid warnings.

activesupport/lib/active_support/core_ext/class/attribute.rb

@@ -75,6 +75,7 @@ def self.#{name}=(val)
           val
         end
 
+        remove_method :#{name} if method_defined?(:#{name})
         def #{name}
           defined?(@#{name}) ? @#{name} : singleton_class.#{name}
         end