Return a StrictRedis instance or None based on redis_spec.

        redis_spec may be None, a Redis URL, or a StrictRedis instance

def _init_redis(redis_spec):
        """
        Return a StrictRedis instance or None based on redis_spec.

        redis_spec may be None, a Redis URL, or a StrictRedis instance
        """
        if not redis_spec:
            return
        if isinstance(redis_spec, six.string_types):
            return redis.StrictRedis.from_url(redis_spec)
        # assume any other value is a valid instance
        return redis_spec