Decorate responses from Rack Middleware and Rails Metal for the purposes of integration testing. A test for the following Metal:

    class Poller < Rails::Rack::Metal
      def call(env)
        if env["PATH_INFO"] =~ /^\/poller/
          [200, {"Content-Type" => "text/plain"}, "Hello World!"]
        else
          super
        end
      end
    end

might be tested like so:

  class PollerTest < ActionController::IntegrationTest
    test "poller returns hello world" do
      get "/poller"
      assert_response 200
      assert_response :success
      assert_response :ok
      assert_equal "Hello World!", response.body
    end
  end

[#1588 state:committed]
Signed-off-by: NDavid Heinemeier Hansson <david@loudthinking.com>