def app
      lambda do |env|
        request = Request.new(env)
        response = router.call(request)

        response[-1] = Array(response[-1])
        response
      end
    end
def app
      lambda do |env|
        request = Request.new(env)
        response = router.call(request)

        response[-1] = Array(response[-1])
        response
      end
    end
The actual application the server will respond to.

 @return [RackApp]