Fix problems with namespaced models and radio buttons

Current master with this code:

    <%= form_for Admin::User.new do |f| %>
      <%= f.radio_button active, 'true' %>
      <%= f.radio_button active, 'false' %>
    <% end %>

will produce:

    <label for="admin::user_active_true">True<label> 
    <label for="admin::user_active_false">False</label> 

This is caused by the object name being in a namespace (the Admin:: part). This problem is fixed by using the object name derived by Rails itself, which is exactly what this pull_request does.