Used relationship name in the naming of creation methods for has_one

@seancarr 

Tested in db_schemas. Only affects 2 classes and effected methods in these classes weren't being used.

I'm not entirely satisfied with this solution. Would prefer to change some members in AssociationDefn, but I don't know the full implications of such changes. This change is small, simple, and gets us the desired behavior.
Variable name should be "association_name" or something like that. I think this is how create() should have been defined the whole time.