'Ensure the Bearer token is valid and authorized access to scopes.
OBS! The request.user attribute should be set to the resource owner
associated with this refresh token.
:param refresh_token: Unicode refresh token
:param client: Client object set by you, see authenticate_client.
:param request: The HTTP Request (oauthlib.common.Request)
:rtype: True or False
Method is used by:
- Authorization Code Grant (indirectly by issuing refresh tokens)
- Resource Owner Password Credentials Grant (also indirectly)
- Refresh Token Grant'

def validate_refresh_token(self, refresh_token, client, request, *args, **kwargs):

    raise NotImplementedError(u'Subclasses    must    implement    this    method.')
