Refactor Session::Cookies::InstanceMethods

Fix the two complex methods pointed out by Code Climate: https://codeclimate.com/github/binarylogic/authlogic/Authlogic::Session::Cookies::InstanceMethods

My changes:

- extract local variable `cookie` within `cookie_credentials`
- extract method `generate_cookie_for_saving` from within `save_cookie`

Code Climate also points out a duplication issue with `timeout.rb`, but it is a false positive.