Router Service: Behaves differently in engine context regarding route path

Hey there,

it seems like the router services behaves differently in an engine context compared to `{{#link-to}}` in views and `transitionTo` routes.

Example: I want to link to my engine's route called 'overview'. The engine is mounted as 'engine'.

This is the expected and documented behavior:
Route: `this.transitionTo('overview')`
Views: `{{#link-to 'overview'}}Link{{/link-to}}`

Whereas the service requires the mount point additionally:
Service: `this.routerService.transitionTo('engine.overview')`

I think all three ways should require the same route path.

Versions:
- `ember-source: ~3.2.0`
- `ember-engines: ~0.5.4`

Hopefully, there's no other issue like that, but haven't found any while searching. Thanks!