fix: only check iat if nbf is not used (#493)

Author: croensch

src/JWT.php

@@ -156,7 +156,7 @@ public static function decode(
         // Check that this token has been created before 'now'. This prevents
         // using tokens that have been created for later use (and haven't
         // correctly used the nbf claim).
-        if (isset($payload->iat) && $payload->iat > ($timestamp + static::$leeway)) {
+        if (!isset($payload->nbf) && isset($payload->iat) && $payload->iat > ($timestamp + static::$leeway)) {
             throw new BeforeValidException(
                 'Cannot handle token prior to ' . \date(DateTime::ISO8601, $payload->iat)
             );