authenticate_user! action must return user

Called by authenticate_user_with_basic_auth!
This requires the user to be returned for further action.

Fixes datacite/datacite#973

app/controllers/application_controller.rb

@@ -77,6 +77,7 @@ def authenticate_user!
 
     @current_user = User.new(credentials, type: type)
     fail CanCan::AuthorizationNotPerformed if @current_user.errors.present?
+    @current_user
   end
 
   def current_ability