scope is optional in code response_type?

Hi!

When i read the doc in **_oauthlib/oauth2/rfc6749/grant_types/authorization_code.py**_, i can see :
`scope
                OPTIONAL.  The scope of the access request as described by
`Section 3.3`_.`

But this check assume that scopes is mandatory
`
            if not request.scopes:
                raise ValueError('Scopes must be set on post auth.')`

Am i misunderstanding something? 
