OAuth2Provider does not use an expiration time when caching the state

The [OAuth1Provider](https://github.com/mohiva/play-silhouette/blob/master/app/com/mohiva/play/silhouette/core/providers/OAuth1Provider.scala#L83) uses an expiration time from 10 minutes when caching the request token. But the [OAuth2Provider](https://github.com/mohiva/play-silhouette/blob/master/app/com/mohiva/play/silhouette/core/providers/OAuth2Provider.scala#L92) doesn't use an expiration time. In SecureSocial this is equally implemented. Is there a reason for this?