'Prepare an access token request, using a refresh token.
If the authorization server issued a refresh token to the client, the
client makes a refresh request to the token endpoint by adding the
following parameters using the "application/x-www-form-urlencoded"
format in the HTTP request entity-body:
grant_type
REQUIRED.  Value MUST be set to "refresh_token".
refresh_token
REQUIRED.  The refresh token issued to the client.
scope
OPTIONAL.  The scope of the access request as described by
Section 3.3.  The requested scope MUST NOT include any scope
not originally granted by the resource owner, and if omitted is
treated as equal to the scope originally granted by the
resource owner.'

def prepare_refresh_body(self, body=u'', refresh_token=None, scope=None, **kwargs):

    refresh_token = (refresh_token or self.refresh_token)
    return prepare_token_request(u'refresh_token', body=body, scope=scope, refresh_token=refresh_token, **kwargs)
