partial_updates clobbers serialized attributes (PATCH)

Ref: http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/788

Hi Rail Core,
     It seems that partial_updates causes serialized attribute to be
skipped when saving (unless *_will_change! is called first).

This causes previously working 2.0 apps to fail in none obvious,
silent ways in 2.1.

I've written a minimalist patch (see ticket) to resolve this (and only
this) and would greatly appreciate any feedback.

Thanks
  - Tom Lea

Long time fan / First time poster