Add route_defined? (private) method

Current edge Rails split named_routes.helpers to path_helpers and url_helpers.
And add `route_defined?` method to NamedRouteCollection.
Because of it, Controller Spec is failed on edge Rails.
This patch fixes it.

refs.
- https://github.com/rails/rails/commit/f889831ed65bea14b6b687bdaa4012d73d81b2a6
- https://github.com/rails/rails/commit/210b338db20b1cdd0684f40bd78b52ed16148b99