Retain refresh token expiry on refreshing (googleworkspace#360)

* fix refresh token expiry lost during refresh

* add failing test

* revert previous changes to go with a different approach

* stab at expires at variables

* fix test and typo

* fix typo

* clean up code for easier review, minimize changes, follow previus patterns

* add missing var

* fix canRefresh logic

* add setExpiresAt tests

* remove extra comment

* add tests and fix typo in logic

Co-authored-by: Steve Bazyl <sbazyl@google.com>

Author: chrisirhc