Breaking test: resetNamespace _within_ an engine's routes is broken

[machty]

Using resetNamespace:true when mounting an engine works, but if the engine itself has routes with resetNamespace:true, those routes get entirely stripped of their engine mount prefix, causing errors within link-to and any other place that needs to resolve the route name.

In this PR, post-reset-namespace should have a route name of blog.post-reset-namespace but it gets registered (potentially multiple times if there are multiple mounts) as just post-reset-namespace, causing errors like:

Error: Assertion Failed: You attempted to generate a link for the "post-reset-namespace" route, but did not pass the models required for generating its dynamic segments. There is no route named blog.post-reset-namespace

I suspect the fix might need to be in Ember proper, but this seemed like the right place to share.

[runspired]

I think I agree here, resetNamespace should consider the current engine (the root app in the case of an app, which is in fact also implemented as an engine!) as the root, not the app root. I believe this would be a fix in either router.js or ember-source.

Not fixing this leaks the isolation primitive and results in a high potential for conflict, honestly just not great all around. So hopefully we can do that.