supporting partial updates for serialized attributes

Hi,

I'd just like some feedback or suggestions on my patch which adds
support for partial updates on serialized attributes:
https://rails.lighthouseapp.com/projects/8994/tickets/2764

It stores the serialized yaml into a hash when loading the record. On
save it checks the serialized values of the current attributes against
the original to see if they changed or not.

I created this because our application depends on the serialized
attributes of our records not being clobbered.

- steve