Fix TypeError in credentials_lib
Fix TypeError in credentials_lib

This fixes an error introduced with lint fixing where computing the
token_expiry of a credential with an expires_in value would result
in an exception.

cherba/apitools,betamos/apitools,thobrla/apitools,houglum/apitools,google/apitools,pcostell/apitools,b-daniels/apitools,kevinli7/apitools,craigcitro/apitools