fix add_namespace methods in case of nested array args

[lastcat]

redis_namespace unsupported hmset of array args.
ex:

redis = Redis.new
namespaced = Redis::Namespace.new(:named, redis: redis)

namespaced.hmset(["key","hoge","fuga","piyo","poyo"])
puts namespaced.hgetall("key")
redis.hmset(["key","hoge","fuga","piyo","poyo"])
puts redis.hgetall("key")

output is

{"named:hoge"=>"named:fuga", "named:piyo"=>"named:poyo"}
{"hoge"=>"fuga", "piyo"=>"poyo"}

The unexpected behavior reason why is that add_namespacemethod is not considered about nested array args. But redis-rb gem support it. I think interface between these gems are same is better, so added flatten process for nested array args. It is confirmed that this change is to pass all tests.