Added possibility to pass objects when working with Identity Associations

@geemus What do you think of this? It would make easier to work with providers that does not return models, by accepting both the identity and the object when setting the value like:

```ruby
@model = MyModel.new
@association = @conn.associations.get('some-id')
@model.association = @association
# or
@model.association = @association.identity
```