Copy over INSTANCE_TT when duping class

src/kernel.c

@@ -285,6 +285,7 @@ copy_class(mrb_state *mrb, mrb_value dst, mrb_value src)
   }
   dc->mt = kh_copy(mt, mrb, sc->mt);
   dc->super = sc->super;
+  MRB_SET_INSTANCE_TT(dc, MRB_INSTANCE_TT(sc));
 }
 
 static void

test/t/kernel.rb

@@ -221,6 +221,14 @@ assert('Kernel#dup', '15.3.1.3.9') do
   assert_false c.respond_to?(:test)
 end
 
+assert('Kernel#dup class') do
+  assert_nothing_raised do
+    Array.dup.new(200)
+    Range.dup.new(2, 3)
+    String.dup.new("a"*50)
+  end
+end
+
 # Kernel#eval is provided by mruby-eval mrbgem '15.3.1.3.12'
 
 assert('Kernel#extend', '15.3.1.3.13') do