Show routes defined in under assets prefix

I want to define my application routes under `assets.prefix`.

* `config/application.rb`

``` ruby
module Demo
  class Application < Rails::Application
    config.assets.prefix = '/demo'
  end
end
```

* `config/routes.rb`

``` ruby
Demo::Application.routes.draw do
  scope '/demo' do
    resource :users
  end
end 
```

In this case, `$ rake routes` couldn't show routes I defined.
So I modified condition for internal routes.

If this PR is acceptable, I'll add test case.