Mounted Rack apps should have default named routes based on app name

This fixes a regression in 4.2.0 from 4.1.8, introduced in 4a7b95985.

#17823 fixed a similar regression regarding _explicitly_ named routes for a mounted Rack app, but there was another regression for the default value.

With a route like:

```ruby
Rails.application.routes.draw do
  mount Mountable::Web, at: 'some_route'
end
```

The "Prefix" column of rake routes gives the following:

```
- 4.1.8:         mountable_web
- 4.2.0.beta1-4: [nothing]
- 4.2.0.rc1:     [nothing]
- 4.2.0.rc2:     some_route   <- regression
```

This fixes the default to go back to being based off the name of the class [like the docs specify](https://github.com/rails/rails/blob/785d04e3109f69d0b9b9f4732179592f0ef04e52/actionpack/lib/action_dispatch/routing/mapper.rb#L558-L560)

Explicitly named routes still work correctly per #17823:

```ruby
Rails.application.routes.draw do
  mount Mountable::Web, at: 'some_route', as: 'named'
end
```

```
- 4.1.8:         named
- 4.2.0.beta1-4: [nothing]
- 4.2.0.rc1:     [nothing]
- 4.2.0.rc2:     named
```

/cc @rafaelfranca @tenderlove