Rails3: use .to_s for overriding human_attribute_name parameter at group model

On Rails 3.0.11, parameter is symbol not string in activerecord validation.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@8969 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/group.rb

@@ -52,7 +52,7 @@ class Group < Principal
   end
 
   def self.human_attribute_name(attribute_key_name, *args)
-    attr_name = attribute_key_name
+    attr_name = attribute_key_name.to_s
     if attr_name == 'lastname'
       attr_name = "name"
     end