Fix Devise::ConfirmationsController#after_confirmation_path_for

after_confirmation_path_for checks whether the user already signed in
by calling signed_in? after confirmation succeeded.
Since it was called without scope specification, the user treated as
signed in inappropriately when the user signed in as another resource
(such as 'admin').
Merge pull request #3004 from kento1218/topic/confirmation-success-redirect

Fix Devise::ConfirmationsController#after_confirmation_path_for