Fix that `Module#to_s` may return frozen string; ref
MODIFY
src/class.c
src/class.c
@@ -1885,7 +1885,8 @@ mrb_mod_to_s(mrb_state *mrb, mrb_value klass)
return mrb_str_cat_lit(mrb, str, ">");
}
else {
- return class_name_str(mrb, mrb_class_ptr(klass));
+ mrb_value str = class_name_str(mrb, mrb_class_ptr(klass));
+ return mrb_frozen_p(mrb_basic_ptr(str)) ? mrb_str_dup(mrb, str) : str;
}
}

Fix that `Module#to_s` may return frozen string; ref 08eafe2