call initialize_copy.

* object.c (rb_obj_dup): call initialize_dup hook.

* lib/delegate.rb (Delegator#initialize_clone): use new hook to
  implement deep copy.  [ruby-dev:40242]

* lib/delegate.rb (Delegator#initialize_dup): ditto.

* test/test_delegate.rb (TestDelegateClass#test_copy_frozen): add
  a test to ensure #clone copies frozen status.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@26620 b2dd03c8-39d4-4d8f-98ff-823fe69b080e