Add serialized capacity to Cache_Redis for objects

Updated with Tcholakov's suggestion.
Verified on Both Mac and Ubuntu environments and run correctly with my database result.
These checks are needed.

What would happen without this check:
You have stored a serialized value 'data'. Then you apply decrement on 'data'. The result should be false and 'prefix:data' stay intact. But instead of this a new value -1 under the key 'data' will appear, and the value 'prefix:data' will get "orphaned".

For the operations increment and decrement Redis assumes non-existent values as zero.