fix use_attr_accessible?

Author: mceachen

lib/closure_tree/support.rb

@@ -23,7 +23,8 @@ def connection
     def use_attr_accessible?
       ActiveRecord::VERSION::MAJOR == 3 &&
         defined?(ActiveModel::MassAssignmentSecurity) &&
-        model_class.accessible_attributes.empty?
+        model_class.respond_to?(:accessible_attributes) &&
+        model_class.accessible_attributes.present?
     end
 
     def include_forbidden_attributes_protection?