Rack::OAuth2::Server.token_for returns an expired token

[DanielVartanov]

When token is expired, there is no way to re-new it. A request to /oauth/access_token returns the same access token.

An easy way to reproduce the bug:

Rack::OAuth2::Server.token_for(user.id.to_s, client.id.to_s, client.scope, 1.second)
# Wait for 1 second :-)
Rack::OAuth2::Server.token_for(user.id.to_s, client.id.to_s, client.scope)

expected: Either another token or the same token but with updated #expires_at property
actual: The same token with the same properties. An attempt to use this token responds with "The access token has expired."