object.c: rb_obj_copy_ivar

* object.c (rb_obj_copy_ivar): extract function to copy instance
  variables only for T_OBJECT from init_copy.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46501 b2dd03c8-39d4-4d8f-98ff-823fe69b080e