Deduce Devise scope from _current user. Fixes #1386, partly.

If you configure Devise with sign_out_all_scopes = false,
and authenticate with a scope that's different from the default one (like :admin) the sign out link did not work. This fixes that.

Unfortunately i could not figure out how to add a helper spec for this. So i added a new integration spec to ensure that things do not explode.