added StrictRedis support

redis-py client has two implementations: Redis and StrictRedis, which differ slightly in their semantics (code written for StrictRedis might not pass the tests using Redis, for example, if ZADD method is used).
A 'strict' boolean flag is added to redisdb fixture to support both implementations.
Since StrictRedis is the recommended implementation, it is set by default.
please extend the docblock with **strict** description