'Dummy client used when an invalid client key is supplied.
:returns: The dummy client key string.
The dummy client should be associated with either a client secret,
a rsa key or both depending on which signature methods are supported.
Providers should make sure that
get_client_secret(dummy_client)
get_rsa_key(dummy_client)
return a valid secret or key for the dummy client.
This method is used by
* AccessTokenEndpoint
* RequestTokenEndpoint
* ResourceEndpoint
* SignatureOnlyEndpoint'

@property
def dummy_client(self):

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