Merge pull request #2663 from scaryzet/fix_expire_session_data_after_sign_in

Fix expire_session_data_after_sign_in!

lib/devise/controllers/helpers.rb

@@ -258,6 +258,10 @@ module Devise
       end
 
       def expire_session_data_after_sign_in!
+        # session.keys will return an empty array if the session is not yet loaded.
+        # This is a bug in both Rack and Rails.
+        # A call to #empty? forces the session to be loaded.
+        session.empty?
         session.keys.grep(/^devise\./).each { |k| session.delete(k) }
       end