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='', refresh_token=None, scope=None, **kwargs):
        """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.
        """
        refresh_token = refresh_token or self.refresh_token
        return prepare_token_request(self.refresh_token_key, body=body, scope=scope,
                                     refresh_token=refresh_token, **kwargs)