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