remove non-column attributes on create

rails/rails@fb2a1c4b removed a check for "real" columns from the `attributes_for_update` method.
In Rails 4.2, default values for attributes that are not actual columns
would be sent to the database.  Rails < 4.2 would remove these columns.

This patch removes non-columns from the default values list befor
sending it to Rails.
Merge pull request #49 from tenderlove/attrbute_methods

remove non-column attributes on create