Dup does not dup translations (KeyValue backend)

[shioyama]

#84 fixed this issue for the Table backend, but it is still broken for the KeyValue backend.

Context

dup does not duplicate translations. Suppose we have:

post = Post.new
post.title = "foo"
post.save
post.title
#=> "foo"
post_dup = post.dup

Expected Behavior

The title of the dup'ed post should be "foo":

post_dup.title
#=> "foo"

Actual Behavior

The title is nil, because the translations are not dup'ed:

post_dup.title
#=> nil

There is already a spec for this which is disabled for KeyValue backends (AR + Sequel), so if someone works on this, just remove that skip code and make the test pass.

[shioyama]

Maybe helpful: https://stackoverflow.com/questions/26287699/rails-deep-duplicate-a-record-with-polymorphic-children