shift(nanotime, fill=NA) does not fill nanotime NA

[mschubmehl]

Calling shift on a nanotime does not identify the nanotime as inheriting from bit64::integer64, and thus fills the numeric NA instead of the integer64:

data.table::shift(nanotime::nanotime(1:4))

[1] "2262-02-18T20:47:17.227407266+00:00" "1970-01-01T00:00:00.000000001+00:00"
[3] "1970-01-01T00:00:00.000000002+00:00" "1970-01-01T00:00:00.000000003+00:00"

Fixed by #3942:

[1] NA                                    "1970-01-01T00:00:00.000000001+00:00"
[3] "1970-01-01T00:00:00.000000002+00:00" "1970-01-01T00:00:00.000000003+00:00"