Updating does not fire callbacks with updated attributes

When updating a record via `upsert!`, callbacks do not use the latest attribute values. See example below:

```ruby
class Car < ActiveRecord::Base
upsert_keys [:internal_id]

before_save :calc_sum

def calc_sum
self.sum = first_val + second_val
end
end

Car.upsert!(internal_id: 1, first_val: 1, second_val: 2) # sets sum to 3

car = Car.upsert!(internal_id: 1, first_val: 2, second_val: 2) # *should* set sum to 4

car.sum == 4 # returns false, sum is still 3
```

I think its related to [this commit](https://github.com/jesjos/active_record_upsert/commit/6387036bf91397cd6203cc7c279d8c7b76020771).

Start with the upsert! implementation and inspect the commit linked in the issue, then reproduce the provided Car example. Done means the before_save callback sees the updated attributes and the second upsert produces sum == 4; run the existing test suite and add coverage where the current behavior is exercised.

Written by the indexing model from the issue text.