Send object a message instead of directly setting the attribute

This line will lead to fail when you have nested attributes in a document.

Since the access to the property is made directly (throgh the #[] method) mongoid will store the association_attributes as a property of the document, instead of triggering the nested attributes generated method.

This little patch fixes this issue.
Merge pull request #1 from Pickachu/patch-1

Send object a message instead of directly setting the attribute