fix: validation error with iat

[kimzuni]

Fixes #98

This also fixes a validation error with iat that occurred when setting the schema.

Also,

If the payload returned from jwtVerify includes exp or nbf, those values are generally expected to be numbers—just like iat.

Since the payload type used in jwt or jwt.sign is defined separately, and the validator exists purely for validation, wouldn't it make sense to set the exp and nbf fields in the validator schema to number?