Test without having to stub out the clear_session_user

Devise seems to be stubbing out request.env['warden'] but not
env['warden'], so we can't just stub out the REMOTE_USER without
changing the current way we access warden.  I'm hesitant to do that.
see
https://groups.google.com/forum/?fromgroups#!topic/plataformatec-devise/ewzmA3jYUrg