Fix for getting around mass assignment whitelist.

[hoverlover]

This fixes a problem that occurs when config.active_record.whitelist_attributes = true in config/application.rb. If this is set, preferences tries to build a new Preference object using

attributes = {:name => name, :group_id => group_id, :group_type => group_type}
stored_preferences.build(attributes)

This will fail due to the whitelist configuration option.