Remove session to allow with_routing to be called twice.

Fixes: https://github.com/rails/rails/issues/16814

actionpack/lib/action_dispatch/testing/integration.rb

@@ -326,6 +326,10 @@ def reset!
         @integration_session = Integration::Session.new(app)
       end
 
+      def remove! # :nodoc:
+        @integration_session = nil
+      end
+
       %w(get post patch put head delete cookies assigns
          xml_http_request xhr get_via_redirect post_via_redirect).each do |method|
         define_method(method) do |*args|

actionpack/test/abstract_unit.rb

@@ -194,6 +194,7 @@ def with_routing(&block)
     yield temporary_routes
   ensure
     self.class.app = old_app
+    self.remove!
     silence_warnings { Object.const_set(:SharedTestRoutes, old_routes) }
   end
 

actionpack/test/controller/integration_test.rb

@@ -814,3 +814,39 @@ def app
     assert_response :ok
   end
 end
+
+class IntegrationWithRoutingTest < ActionDispatch::IntegrationTest
+  class FooController < ActionController::Base
+    def index
+      render plain: 'ok'
+    end
+  end
+
+  def test_with_routing_resets_session
+    klass_namespace = self.class.name.underscore
+
+    with_routing do |routes|
+      routes.draw do
+        namespace klass_namespace do
+          resources :foo, path: '/with'
+        end
+      end
+
+      get '/integration_with_routing_test/with'
+      assert_response 200
+      assert_equal 'ok', response.body
+    end
+
+    with_routing do |routes|
+      routes.draw do
+        namespace klass_namespace do
+          resources :foo, path: '/routing'
+        end
+      end
+
+      get '/integration_with_routing_test/routing'
+      assert_response 200
+      assert_equal 'ok', response.body
+    end
+  end
+end