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
I figured `expires_at` could be nil, and nil doesn't respond to that method... Don't know why I didn't just go with a simple if check :grin: