Previously there was no way to get a ISO 8601 timestamp into a specific
timezone without either using `parse` or chaining methods. The new method
allows parsing directly into the timezone, e.g:
>> Time.zone = "Hawaii"
=> "Hawaii"
>> Time.zone.iso8601("1999-12-31T14:00:00Z")
=> Fri, 31 Dec 1999 14:00:00 HST -10:00
If the timestamp is a ISO 8601 date (YYYY-MM-DD) then the time is set
to midnight, e.g:
>> Time.zone = "Hawaii"
=> "Hawaii"
>> Time.zone.iso8601("1999-12-31")
=> Fri, 31 Dec 1999 00:00:00 HST -10:00
This new method has stricter semantics than the current `parse` method
and will raise an `ArgumentError` instead of returning nil, e.g:
>> Time.zone = "Hawaii"
=> "Hawaii"
>> Time.zone.iso8601("foobar")
ArgumentError: invalid date
>> Time.zone.parse("foobar")
=> nil
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