DATAREDIS-251 - Fix tests to work against Redis 2.8.x.

Since Redis 2.8.x keyOps#getExpire() now returns -2 for empty collections (RedisList/RedisSet/RedisMap) we add a dummy entry to keep the test version agnostic. Previously getExpire() returned -1.

For reference see the discussion in http://redis.io/commands/ttl