In ISO datestyle, never emit just HH:MM, always emit HH:MM:SS or

In ISO datestyle, never emit just HH:MM, always emit HH:MM:SS or
HH:MM:SS.SSS... when there is a nonzero part-of-a-day field in an
interval value.  The seconds part used to be suppressed if zero,
but there's no equivalent behavior for timestamp, and since we're
modeling this format on timestamp it's probably wrong.  Per complaint
and patch from Larry Rosenman.