During refresh grant type, prevAccessToken is NULL

[pjebs]

https://github.com/felipeweb/osin-mysql/blob/master/mysql.go#L281

In LoadAccess, prevAccessToken is initially ""

Once accessToken is found, then LoadAccess is called again but this time code is "" due to prevAccessToken being null in the database.

Due to this, ErrNotFound is returned the second time LoadAccess is called.

This causes the refresh_token grant type to fail when tokenHandler is called.

Any idea how I can fix this?

[pjebs]

See #17 (please review but it seems to work for me)