Add a test_token() method (Enchancement)

Usefull to make a simple test with current token.

In other apps user could use it and if failed make a xee.get_token_from_refresh_token(saved_refreshed_token) or xee.get_token_from_code(authorization_code)

In all case devs can can make test in app with the store token_ expired_date, or catch the error in every requests.
