use `Gitlab::Routing.url_helpers` instead of `Rails.application.routes.url_helpers`

since `Rails.application.routes.url_helpers` creates a new anonymous module every time it's called

lib/gitlab/middleware/read_only.rb

@@ -58,7 +58,7 @@ module Gitlab
       end
 
       def last_visited_url
-        @env['HTTP_REFERER'] || rack_session['user_return_to'] || Rails.application.routes.url_helpers.root_url
+        @env['HTTP_REFERER'] || rack_session['user_return_to'] || Gitlab::Routing.url_helpers.root_url
       end
 
       def route_hash

spec/lib/gitlab/middleware/read_only_spec.rb

@@ -91,6 +91,12 @@ describe Gitlab::Middleware::ReadOnly do
       expect(subject).to disallow_request
     end
 
+    it 'returns last_vistited_url for disallowed request' do
+      response = request.post('/test_request')
+
+      expect(response.location).to eq 'http://localhost/'
+    end
+
     context 'whitelisted requests' do
       it 'expects a POST internal request to be allowed' do
         expect(Rails.application.routes).not_to receive(:recognize_path)