when :user option is passed.
Tests do not pass under rails 2.3 because of an issue related to respond_to_missing? and delegation. see https://rails.lighthouseapp.com/projects/8994/tickets/5410-multiple-database-queries-when-chaining-named-scopes-with-rails-238-and-ruby-192
This allow us to add custom rules in any part of an application, for example a rails engine. Also do not raise an exception if there isn't a auth rule file under rails root config directory.