Allow revoked_at to be set in the future, for future expiry

Hello,

I'm working with IFTTT integration on a project and they've requested that a refresh token is not revoked immediately. I have no idea how to implement this in doorkeeper without setting `revoked_at` to a future time. I noticed that the `revoked?` method merely checks if that field is set, and not if that time has yet passed. This PR attempts to rectify that behaviour.

If you know of a better way to keep refresh tokens around, I would love to hear about it.

Thanks!