Millisecond precision for iat claim

[Rahil Bohra]

It looks like the fix for issue #35 removed millsecond precision to keep in line with the JWT spec at the time. AFAICT the spec no longer specifies any precision. I think millisecond precision is preferable, and #35 should be reverted.

[Connect2id OSS]

The time unit is strictly defined as seconds for all numeric dates, else interop will turn out to be be a disaster :)

The NumericDate definition is towards the bottom of the spec:

NumericDate
      A JSON numeric value representing the number of seconds from 1970-
      01-01T00:00:00Z UTC until the specified UTC date/time, ignoring
      leap seconds.  This is equivalent to the IEEE Std 1003.1, 2013
      Edition [POSIX.1] definition "Seconds Since the Epoch", in which
      each day is accounted for by exactly 86400 seconds, other than
      that non-integer values can be represented.  See RFC 3339
      [RFC3339] for details regarding date/times in general and UTC in
      particular.