Call `#initialize_copy` from `init_copy` only if it's redefined.

src/kernel.c

@@ -283,6 +283,8 @@ copy_class(mrb_state *mrb, mrb_value dst, mrb_value src)
   MRB_SET_INSTANCE_TT(dc, MRB_INSTANCE_TT(sc));
 }
 
+static mrb_value mrb_obj_init_copy(mrb_state *mrb, mrb_value self);
+
 static void
 init_copy(mrb_state *mrb, mrb_value dest, mrb_value obj)
 {
@@ -310,7 +312,9 @@ init_copy(mrb_state *mrb, mrb_value dest, mrb_value obj)
     default:
       break;
   }
+  if (!mrb_func_basic_p(mrb, dest, MRB_SYM(initialize_copy), mrb_obj_init_copy)) {
     mrb_funcall_id(mrb, dest, MRB_SYM(initialize_copy), 1, obj);
+  }
 }
 
 /* 15.3.1.3.8  */