The former is overtly forbidden by ISO 8601 but should be recognized
due to RFC 3339's use of it. The "Serialising Extended Data About
Times and Events Working Group" (sedate WG) has established how to
resolve this, so document that conclusion and note the consequent
inadvisability of using UTC-00:00 as a zone ID (although it should
still be accepted).
Change-Id: Ib9fbbe6765117bfa9a84e726d0e75f7397a4c813
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>