Dup fails on model utilizing a "self join" has_many

I'm trying to use deep_cloneable on a model that has a self join has_many. Similar to the following
```ruby
class Part < ActiveRecord::Base
  has_many :child_parts, :class_name => 'Part', :foreign_key => 'parent_part_id'
end
```
The error is NoMethodError: undefined method `each' at lib/deep_cloneable.rb:122

I'm unsure how to fix it so this pull request is just a failing test demonstrating the bug.

Thanks.