Fix warnings.

Make sure we don't redefine an already-defined attribute method.

activerecord/lib/active_record/attribute_methods.rb

@@ -36,7 +36,7 @@ def instance_method_already_implemented?(method_name)
 
         @@_defined_activerecord_methods ||= defined_activerecord_methods
         raise DangerousAttributeError, "#{method_name} is defined by ActiveRecord" if @@_defined_activerecord_methods.include?(method_name)
-        @_defined_class_methods.include?(method_name)
+        @_defined_class_methods.include?(method_name) || generated_attribute_methods.method_defined?(method_name)
       end
 
       def defined_activerecord_methods