Add method to format AbsoluteDate according to RFC3339
RFC3339 is a restricted subset of ISO 8601 that is commonly used in internet applications. See https://tools.ietf.org/html/rfc3339#page-8
Instead of adding a new method
AbsoluteDate.toString(*) could be modified to emit RFC3339 format. It may be possible to do #637 at the same time.
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