'Dummy request token used when an invalid token was supplied.
:returns: The dummy request token string.
The dummy request token should be associated with a request token
secret such that get_request_token_secret(.., dummy_request_token)
returns a valid secret.
This method is used by
* AccessTokenEndpoint'

@property
def dummy_request_token(self):

    raise self._subclass_must_implement(u'dummy_request_token')
