Fix for `exp` claim considered valid if equal to now

[jimmyjames]

As discussed in #646, the current date/time must be before the exp claim's value.

From the spec, section 4.1.4 (emphasis added):

The processing of the "exp" claim requires that the current date/time MUST be before the expiration date/time listed in the "exp" claim.

This change ensures that the current time is before the exp claim's value (if present).

Fixes #646