Returns True if the given schema is already registered under the
        given subject.

def schema_is_registered_for_subject(self, subject, schema):
        """
        Returns True if the given schema is already registered under the
        given subject.
        """
        data = json.dumps({'schema': json.dumps(schema)})
        res = requests.post(self._url('/subjects/{}', subject), data=data, headers=HEADERS)
        if res.status_code == 404:
            return False
        raise_if_failed(res)
        return True