WIP: duping leaks dirty tracking to original object

[doits]

hey @shioyama,

I think I just stumbled upon a bug in dirty tracking: duping an object and changing the dupe invalidates/leaks into the dirty tracking of the original object.

I added a failing spec here with what I think should be the right outcome. I'm not 100 % sure about the last assertion about the dupe (expect(dupe.previous_changes).to include({ "title_en" => ["foo", "bar"]})), but the rest seams clear to me.

To reproduce, simply create a record, .dup it and then change the dupe's localized attributes (see the failing spec).

I didn't investigate it yet (could work around it in my app), just want to give you a heads up. Don't know either if this is only a AR thing (were I added the spec) or some general problem with other ORMs, too.

Probably a cache object/hash does not get reset/duped on .dup and then both records share it.

Maybe you can take a look if you have some time? If not I could do so, too, when I find some time for it.

Thanks!