Resolve "Allow expiry date for JWT tokens to be the zero date"

Related to #46

auth/jwt.go

@@ -100,7 +100,7 @@ func ValidateJWT(jsonWebTokenString string, secretKey string) (JsonWebToken, err
 	}
 
 	// Validate the expiry date
-	if jsonWebToken.ExpiryDate.Before(date_utils.CurrentDate()) {
+	if !jsonWebToken.ExpiryDate.IsZero() && jsonWebToken.ExpiryDate.Before(date_utils.CurrentDate()) {
 		return jsonWebToken, errors.Error("token has expired")
 	}