add a 5s tolerance to iat

oauth.go

···
 		"sub": c.clientId,
 		"aud": authServerUrl,
 		"jti": uuid.NewString(),
-		"iat": time.Now().Unix(),
+		"iat": time.Now().Add(-5 * time.Second).Unix(),
 	}
 
 	token := jwt.NewWithClaims(jwt.SigningMethodES256, claims)