RESTFUL Authentification plugin Functional tests - howto ?

I am trying to write a functional test of my UserController index
action

require File.dirname(__FILE__) + '/../test_helper'
require 'users_controller'

class UsersControllerTest < Test::Unit::TestCase
  def setup
    @controller = UsersController.new
    @request = ActionController::TestRequest.new
    @response = ActionController::TestResponse.new
  end

def test_should_get_index
    login_as (:superadmin)
    get :index
    assert_response :success
    assert_not_nil assigns(:users)
    assert_equal 15, assigns(:users).total_entries

    login_as (:quentin)
    get :index
    assert_response :success
    assert_not_nil assigns(:users)
    assert_equal 6, assigns(:users).total_entries
  end
end

I am using the Restful Authentification plugin, without any problem
running my index action

class UsersController < ApplicationController
before_filter :login_required
  def index
   @roles = ....
  @users = User.display_users(current_user, @roles, params[:page])
...
  end
...
end

on the first login_as (:superadmin), it's OK, current_user is 1, but
when doing the login_as (:quentin), the current_user is NOT modified,
still being 1

I wrote in the login_as in :
module AuthenticatedTestHelper
  # Sets the current user in the session from the user fixtures.
  def login_as(user)
    @request.session[:user_id] = user ? users(user).id : nil
  end
...
end

the @request.session[:user_id] is correctlt modified (user_id 1, then
user_id 2) but current_user in my index action is not... where
could be the problem located ??

thanks a lot to any info !

erwin