* Add {whenISO} to record timestamp in ISO 8601 format in UTC.

ISO 8601 is the standard time format and is easy to parse.

This change assumes users desiring ISO 8016 generally prefer UTC for simplicity.
This results in {whenISO} to be significantly shorter than {when}:
{when}    = "02/Jan/2006:15:04:05 +0000"
{whenISO} = "2006-01-02T15:04:12Z"

Add unit test to verify both, as there was no unit test for {when}.

* Rename {whenISO} to {when_iso}