Prevent duplicate session in auth code grant

The session already exists in the database, so we don't need to save it again. Doing so results in the session used for the auth code hanging around in the database with nothing associated to it, while the access token is associated to a new session caused by the `save()` method creating a duplicate. Fixes #266.
Merge pull request #282 from maknz/master

Prevent duplicate session in auth code grant