[FEATURE ember-routing-bound-action-name] Fix assertion in deprecation.

If the action was in the root of the controller (not in an actions
hash), the lookup would find the actual action function itself,
resulting in us trying to trigger an invalid action (using the functions
toString value).
Merge pull request #4336 from rjackson/fix-deprecated-action-with-bound-lookup

[FEATURE ember-routing-bound-action-name] Fix assertion in deprecation.