Fix invalid RFC 3339 in TimeTagToAny (unreleased new feature)

[benluddy]

Description

TimeTagToAny is an unreleased new option that was added by PR 496:

• feature: Add TimeTagToAny option to produce RFC3339 when decoding time into interface{} #496

Unfortunately, TimeTagToAny can produce strings that are not valid RFC 3339 timestamps through its use of (time.Time).Format, whose signature does not allow it to return errors.

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[benluddy]

I discovered this case while exercising the new options from #496. I think we want to avoid producing invalid RFC 3339 here as the stdlib Time's MarshalText and MarshalJSON implementations now do.

[fxamacker]

LGTM! 👍 Thanks for catching this bug in the new feature before v2.7.0 is released!