Check if root_path is defined with #respond_to? instead of #present (#5022)

When an application does not define a `root`, the method will be
undefined instead of returning a falsey value.
This commit also includes a new test with fake objects that mimic this
behavior.

Related resources:

* 1aab449933 (diff-c1be825bdb5f3160081e41432f83d0d7R278)
* https://github.com/plataformatec/devise/issues/5021

lib/devise/failure_app.rb

@@ -275,7 +275,7 @@ module Devise
     private
 
     def root_path_defined?(context)
-      defined?(context.routes) && context.routes.url_helpers.root_path.present?
+      defined?(context.routes) && context.routes.url_helpers.respond_to?(:root_path)
     end
 
     def rails_5_and_down?

test/failure_app_test.rb

@@ -28,6 +28,27 @@ class FailureTest < ActiveSupport::TestCase
     end
   end
 
+  class FailureWithoutRootPath < Devise::FailureApp
+    class FakeURLHelpers
+    end
+
+    class FakeRoutesWithoutRoot
+      def url_helpers
+        FakeURLHelpers.new
+      end
+    end
+
+    class FakeAppWithoutRootPath
+      def routes
+        FakeRoutesWithoutRoot.new
+      end
+    end
+
+    def main_app
+      FakeAppWithoutRootPath.new
+    end
+  end
+
   class FakeEngineApp < Devise::FailureApp
     class FakeEngine
       def new_user_on_engine_session_url _
@@ -103,6 +124,13 @@ class FailureTest < ActiveSupport::TestCase
       end
     end
 
+    test 'returns to the root path even when it\'s not defined' do
+      call_failure app: FailureWithoutRootPath
+      assert_equal 302, @response.first
+      assert_equal 'You need to sign in or sign up before continuing.', @request.flash[:alert]
+      assert_equal 'http://test.host/', @response.second['Location']
+    end
+
     test 'returns to the root path considering subdomain if no session path is available' do
       swap Devise, router_name: :fake_app do
         call_failure app: FailureWithSubdomain