Allow expires_in value of 0

[neumachen]

👨 🐻 🐷

If expires_in has a value of 0, expires_at gets set. This shouldn't be the case.

Use case:
VKontakte returns a 0 value for expires_in for offline access, see https://vk.com/dev/permissions.

[coveralls]

Coverage increased (+0.01%) to 95.87% when pulling 7b1d7a1 on magicalbanana:expires-in-fix into e51cd0a on intridea:master.

[coveralls]

Coverage increased (+0.01%) to 95.87% when pulling 571b52e on magicalbanana:expires-in-fix into e51cd0a on intridea:master.

[josephpage]

Following the RFC, a 0 value in expires_in means the access_token expires as soon as emitted, and not valid forever.

https://tools.ietf.org/html/rfc6749#section-4.2.2

The lifetime in seconds of the access token. For
example, the value "3600" denotes that the access token will
expire in one hour from the time the response was generated.
If omitted, the authorization server SHOULD provide the
expiration time via other means or document the default value.

[pboling]

Agree with @josephpage , this can be closed. For offline access you might need to set a very long expiry.