Fix exception when mounted rack application is not ancestor of engine.

Hi,

Could you consider to merge this change?
When I mounted rack application which is not ancestor of Rails Engine as below, 

```examples/address_book/config/routes.rb
AddressBook::Application.routes.draw do
  mount Respect::Rails::Engine => "/rest_spec"
  mount DelayedJobWeb => "/delayed_job"
```

I got the following exception.

```
'DelayedJobWeb' must be an ancestor of ::Rails::Engine.
```

So, I add the condition for checking an ancestor of ::Rails::Engine.