Fix hset for Array of Key / Value Pairs

[jvanderen1]

Fixes #310

This resolves an issue where Redis and MockRedis were storing values slightly differently:

With Redis:

require 'redis'
redis = Redis.new

hash = {:name=>"job", :namespace=>"default", :klass=>"Job"}
redis.hset("key", hash.flatten)

redis.hgetall("key")
=> {"name"=>"job", "namespace"=>"default", "klass"=>"Job"}

With MockRedis (with this change):

require 'mock_redis'
mock_redis = MockRedis.new

hash = {:name=>"job", :namespace=>"default", :klass=>"Job"}
mock_redis.hset("key", hash.flatten)

mock_redis.hgetall("key")
=> {"name"=>"job", "namespace"=>"default", "klass"=>"Job"}