Merge pull request #318 from masamitsu-murase/fix_instance_tt

Patch for inheritance of built-in class

src/class.c

@@ -1050,6 +1050,9 @@ mrb_class_new(mrb_state *mrb, struct RClass *super)
     mrb_check_inheritable(mrb, super);
   }
   c = boot_defclass(mrb, super);
+  if (super){
+    MRB_SET_INSTANCE_TT(c, MRB_INSTANCE_TT(super));
+  }
   make_metaclass(mrb, c);
 
   return c;