VerifyExpiresAt returns false when not required and exp is empty

[nrgraham23]

The VerifyExpiresAt function returns false when it is not required and the exp token is an empty string because it returns false if there is not a match in the switch statement: https://github.com/golang-jwt/jwt/blob/main/map_claims.go#L58

Based on the comment on the method and the old version of this method, it seems like this is a bug.

[oxisto]

Sorry for the long delay. Somehow this issue slipped my radar. The function was intentionally changed in this PR: #40

The current implementation is a little bit better in terms of security, because an "empty string" is not valid for exp. According to the RFC, exp must be a date timestamp in a JSON number. Therefore, we decided to return false in the end of the switch statement in case someone is trying to validate a token with exp being anything else but a JSON number as an additional safeguard.