…btains the given expiration time date's timezone correctly instead of using local timezone

…xpirationTime was being incorrectly parsed, assuming the date was in the default JVM timezone instead of reading from the date string what timezone it was.

… the date instead of returning dates in different timezones

Expose to tests the calendar object used by the SimpleDateFormat object of the refreshAccessToken method in the ImpersonatedCredentials class
That way we stop relying on returning dates in different timezones in the expireTime returned by the mockTransportFactory

… asserts

…date-parsing