[ActionMailer] Rename test/test_test to test/test_case_test
Merge pull request #16751 from seuros/rename

[ActionMailer] Rename test/test_test to test/test_case_test
Clean up grammar from #16752 [ci skip]
Allow polymorphic routes with nil when a route can still be drawn

Suppose you have two resources routed in the following manner:

```ruby
resources :blogs do
  resources :posts
end

resources :posts
```

When using polymorphic resource routing like `url_for([@blog, @post])`, and `@blog` is `nil` Rails should still try to match the route to the top-level posts resource.

Fixes #16754