Encode expiration as iso8601 in UTC timezone.

Based on @chancancode's comment here: https://github.com/rails/globalid/commit/39ab83a43a64b339739c6e8c7ab24bb1325204ed#commitcomment-7529613

cc @jeremy @xuchu
If an id has less than a second to expiration, I think it would be okay for it to be considered expired.

I mean Secret Agents get around 6-8 seconds before their messages are self-destructed :trollface: