Wrong property path for Yammer

Yammer return's a access_token with extra data like this and the lifetime is null so there is no lifetime.

```
Array
(
    [access_token] => Array
        (
            [user_id] => 1503941481
            [network_id] => 916484
            [network_permalink] => jcid.nl
            [network_name] => jcid.nl
            [token] => <token>
            [view_members] => 1
            [view_groups] => 1
            [view_messages] => 1
            [view_subscriptions] => 1
            [modify_subscriptions] => 1
            [modify_messages] => 1
            [view_tags] => 1
            [created_at] => 2013/11/12 09:36:49 +0000
            [authorized_at] => 2013/11/12 09:36:49 +0000
            [expires_at] => 
        )
}
```