MessageFactory::createRedirect: generate Resent-Date same way as KMime does for Date

KMime uses QDateTime to generate the date headers, so we need to use the same
way otherwise we get timezone inconsistencies (when system timezone != UTC)
in MessageFactoryTest, because it's using fake XDG_CONFIG_HOME, so KDateTime
does not have access to the configured timezone (Qt uses different way to get
timezones).