Allow routing specs to access engine routes

I have a Rails Engine with routes defined in my app.
After upgrade all routing tests fail with _No route matches..._

I was trying to debug the MyEngine::Engine.routes.router and here's the diff
On latest rspec-rails `@ast` is always nil.

```diff
@@ -1,13 +1,14 @@
 pry(#<RSpec::Core::ExampleGroup::Nested_14::Nested_1>)> @routes.router
-=> #<Journey::Router:0xa1eb388
+=> #<Journey::Router:0xa4e6150
  @options=
   {:parameters_key=>"action_dispatch.request.path_parameters",
    :request_class=>ActionDispatch::Request},
  @params_key="action_dispatch.request.path_parameters",
  @request_class=ActionDispatch::Request,
  @routes=
-  #<Journey::Routes:0xa1eb3ec
-   @ast=nil,
+  #<Journey::Routes:0xa4e61b4
+   @ast=
+    /signup(.:format)|/login(.:format)|/logout(.:format)|/signup(.:format)|/login(.:format)|/start(.:format)|/start(.:format)|/sessions(.:format)|/sessions/new(.:format)|/sessions/:id(.:format)|/passwords(.:form
    @named_routes=
     {"signup"=>
-      #<Journey::Route:0xc685004
\ No newline at end of file
+      #<Journey::Route:0xa6d40c0
\ No newline at end of file
```