Cast f64 NumericDate to i64

[georgeboot]

The JWT spec states the following definition for NumericDate:

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.

This means that numeric dates can be a f64 as well.

This PR casts the f64 to an i64 internally in the deserialisation, so support both.