[3.1.0.rc5] Failing routing for all tests in basic app generation

Hi, I'm trying to generate a mountable engine and have done the

rails plugin new foo --mountable
cd foo
rails generate scaffold bar name:string
rake test

This results in errors for every test such as this:

  4) Error:
NoMethodError: undefined method `bars' for #<Foo::BarsControllerTest:
    actionpack (3.1.0.rc5) lib/action_dispatch/testing/assertions/
routing.rb:175:in `method_missing'
    activesupport (3.1.0.rc5) lib/active_support/callbacks.rb:440:in
    activesupport (3.1.0.rc5) lib/active_support/callbacks.rb:81:in
    activesupport (3.1.0.rc5) lib/active_support/callbacks.rb:81:in
    activesupport (3.1.0.rc5) lib/active_support/testing/
setup_and_teardown.rb:65:in `run'

Ironically, running 'rails server' and going to 'http://localhost:3000/
foo/bars' shows that the engine and all the routing is working
perfectly. But all tests choke on not being able to find 'bars_path'
or the like.

I'm having a similar problem, I'm upgrading an engine to 3.1 and all
my routing specs are red.

Is there a way to debug/print out the generated engine routes? When
running rake routes from my dummy app I can't see the expanded routes,
all I see is mything / {:to=>MyThing::Engine}.

Anything new here? What should one do? The tests look allright...