Enable to use `permits` in controllers with namespace

I implemented folllowing admin function with using action_args and rails3:

#### config/routes.rb
```ruby
namespace :admin do
  resources :articles
end
```

#### app/controllres/admin/articles_controller.rb
```ruby
module Admin
  class ArticlesController < ApplicationController
    permits :title, :body
    def create article
      # default create action generated by scaffold
    end
  end
end
```

But, the `permits` method is not worked because `class_name` on here is `Admin::ArticlesController`, thus `Admin::Article` is called as model name.

This commit reduces namespace of controller when detecting the model name.