hagrid's HKP index output doesn't include expirationdate field
HKP suggests that the machine-readable index form includes an expirationdate
field.
But hagrid returns an empty value for that field for all certificates listed, even those that do have expiration dates.
If there is no specific reason to avoid reporting this data, it would be good to more fully implement HKP by including that field, based on the cryptographically-verified self-sig.