Update experiment settings

Updates settings on a campaign experiment that's in <code>draft</code> or <code>ready</code> status. Iterable changes only the fields you send. Omitted fields keep their current values, and <code>null</code> removes a setting when the field allows it. Blast campaigns accept blast and holdout settings, and triggered campaigns accept trigger settings. This endpoint works only with experiments that test message content: the subject line, preheader text, from name and sender, email body, or all content. It doesn't work with send time or Send Time Decisioning experiments. This endpoint doesn't support journey experiments.