Copy changed_attributes across to newly become'd records

Without this, the original record's values won't get saved, since the partial insertions support (https://github.com/rails/rails/commit/144e8691cbfb8bba77f18cfe68d5e7fd48887f5e) checks for changed values and thinks there are none.

I was running into this when copying an STI record from one subtype to another, with : 

```ruby
foo = Foo.last
bar = foo.dup.becomes(Bar)
bar.save
```
and bar would be missing attributes from Foo.  This problem shows up anytime after https://github.com/rails/rails/commit/144e8691cbfb8bba77f18cfe68d5e7fd48887f5e, so the above worked fine in Rails 3.2