fixed bug where passwords are stored in plaintext

https://bugs.launchpad.net/fuel/+bug/1361796
since it pashword hasing was moved out of on_before_clean and into on_before_save, and in on_before_clean new_password gets set back to password, this should be reflected in on_before_save

without this fix logins will never validate for those new users and -the password is stored as plaintext!-