In mid-transition, `modelFor` accepts both camelCase and underscore naming

With facelift router, `modelFor` maintains backwards compatibility
to return a resolved model on a parent route even in mid-transition.
This PR fixes a bug where only the underscore version of a route name
yielded a value in mid transition.
Merge pull request #2855 from machty/model-for-fix

In mid-transition, `modelFor` accepts both camelCase and underscore naming