Dupping a CollectionProxy should dup the load_target

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

activerecord/lib/active_record/associations/collection_proxy.rb

@@ -33,6 +33,10 @@ def initialize(klass, association) #:nodoc:
         super klass, klass.arel_table, klass.predicate_builder
       end
 
+      def initialize_dup(other) # :nodoc:
+        @association = @association.deep_dup
+      end
+
       def target
         @association.target
       end

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -2107,6 +2107,14 @@ def test_to_a_should_dup_target
     assert_not_equal target.object_id, ary.object_id
   end
 
+  def test_dup_should_dup_load_target
+    original = topics(:first).replies
+    dupped = topics(:first).replies.dup
+
+    assert_not_equal original.object_id, dupped.object_id
+    assert_not_equal original.load_target.object_id, dupped.load_target.object_id
+  end
+
   def test_merging_with_custom_attribute_writer
     bulb = Bulb.new(color: "red")
     assert_equal "RED!", bulb.color