OmniAuth's [Auth Hash Schema] should return an `expires_at` field as a
timestamp, but this gem returns `expires_in`.  For compatibility with
other `oauth2` OmniAuth strategies, we should also return `expires_at`.

I'm not sure if the best place to fix it is here or upstream, in
`Rack::OAuth2::AccessToken`.  On the one hand, the `oauth2` gem handles it
in `OAuth2::AccessToken`.  On the other hand, the OmniAuth strategy is
the only place we can ensure minimal latency between the server response
and `expires_at` computation.  I chose here. 🙂

[Auth Hash Schema]: https://github.com/omniauth/omniauth/wiki/Auth-Hash-Schema

n.b. I would have assumed that "timestamp" in the schema meant a Time
object, but all of the gems that inherit from `omniauth-oauth2` return
`Time#to_i`, which is also appropriate.