fix example typo
Allow model constructors to take already initalized objects

If a constructor argument is an already contructed object
(that is, it's a kind_of of the associated class), simply
use the given object as the association value.

This allows `her` to behave a bit more as expected by
normal rails guys.

```ruby
   User.new(organization: already_fetched_org)
```
Merge pull request #198 from vic/new_can_take_existing_objects

New can take existing objects