Fixed: Pundit's AuthorizationNotPerformedError

[ohrite]

After setting up Pundit on a new application, Pundit's README says that I can ensure that every request is authorized:

class ApplicationController < ActionController::Base
  include Pundit
  after_action :verify_authorized
  ...
end

However, as @Samy-Amar has identified in #2642, Pundit checks an instance variable (@_pundit_policy_authorized) as part of this process, and RailsAdmin's Pundit adapter does not set @_pundit_policy_authorized.

This PR sets @_pundit_policy_authorized, and uses some of the existing Pundit testing infrastructure to introduce a RailsAdmin::MainController-level spec for Pundit authorization.

[mshibuya]

Excellent, thanks!