Integration tests call the whole rack middleware stack, but stubbornly return the last controller response, which can be completely different especially if you use warden or other middleware-tools.

See actionpack-2.3.14/lib/action_controller/integration.rb:342

To fix that and prepare for Rails 3 (which also relies on rack response) do this:

# test/test_helper.rb

if Rails::VERSION::MAJOR == 2
  # https://grosser.it/2012/10/19/rails-2-your-integration-tests-are-lying/
  # make integration tests use rack response, so we can test our middlewares
  # and not only the pure controller response

  ActionController::Base.class_eval do
    # this is usually done just-in-time by #process but we need to do it earlier
    include ActionController::Integration::ControllerCapture

    # then we hide last_instantiation from #process
    def self.last_instantiation;end
  end

  ActionController::Integration::Session.class_eval do
    def process_with_rackify(*args)
      process_without_rackify(*args)
    ensure
      # needed e.g. inside of assert_redirect_to
      capture = ActionController::Integration::ControllerCapture::ClassMethods

      # not set by original #process
      @response.redirected_to = @response.headers["Location"] if @response
      if @controller = capture.send(:class_variable_get, :@@last_instantiation)
        @request = @controller.request
        @response.template = @controller.response.template if @controller.response
        @controller.send(:set_test_assigns)
      end
    end
    alias_method_chain :process, :rackify
  end
end