In Rails 3.2 dupping a `CollectionProxy` would dup it's `load_target` as
well. That functionality has been broken since the release of Rails 4.0.
I hit this in an application upgrade and wondered why duplicating a
CollectionProxy and assigning it to a variable stopped working.

When calling `dup` on a `CollectionProxy` only the owner (ex.
topic) was getting duplicated and the `load_target` would remain in tact
with it's original object ID. Dupping the `load_target` is useful for performing
a logging operation after records have been destroyed in a method.

For example:

```
def transfer_operation
  saved_replies = topic.replies

  topic.replies.clear

  saved_replies.each do |reply|
    user.update_replies_count!
  end
end
```

This change adds a `initialize_dup` method that performs a `deep_dup` on
the `@associatiation` so that the `load_target` is dupped as well.

Fixes #17117