Add Array Capability for Cache_redis.php

Hello, 
Today I tried to use this Cache_redis.php for my application to deal with some query result set which is array-based data. Due to the native features of Redis that does not support 'array' data type. So I add the two 'serialize' and 'unserialize' function to 'save' and 'get' separately to add multidimensional array capability. Wish it can help some people like me who wants to deal with the same array set cache.

Regards,
Chonhan
Update Cache_redis.php
Update Cache_redis.php

Do type checking before `serialize()` / `unserialize()` functions
Update Cache_redis.php

correct typo