[`@embroider/router`] router.urlFor only works when a string is passed

[andrew-paterson]

When using @ember/routing/router as the router, the following pattern works as stated in the API docs:

this.router.urlFor('my-route, {id: 1});
// /my-route/1

After changing to @embroider/router to enable route splitting, this behaviour changes.

this.router.urlFor('my-route, {id: 1});
// /my-route/%5Bobject%20Object%5D

Passing a primitive value does work though:

this.router.urlFor('my-route, 1);
// /my-route/1

[johanrd]

For reference as possibly related:
#2377
#2381