Unable to save foreign key after child loaded

Found an issue where update_attributes doesn't work for foreign key attributes if the foreign child has been loaded. 

``` ruby
a = Attendee.find(12345)
puts a.candidate_type.id # 879
a.update_attributes(candidate_type_id: 880)
a.save # will PUT with 879, not 880
```

Do you think this is an expected behavior or a bug?
