Track changed attributes

[tim-vandecasteele]

Make it possible to use title_en_was.

This duplicates some dirty_tracking code but I didn't see another way.

[tamadamas]

@tim-vandecasteele I tried your solution, but It's wrong for me.
I wrote own solution but without _was feature.
https://github.com/mli-max/globalize-accessors/commit/7fb5307e703008020b95622b262fa39e16d10af0

Your solution fails in my tests with error

Failure/Error: expect(execution_summary.changes[:title_en]).to eq [nil, "New title"]

       expected: [nil, "New title"]
            got: ["Canyoning sur le Rhône", "New title"]

It got from default title value. It's a bit unsuspected but it should be nil like in Rails way

      expect(execution_summary.changes[:title_en]).to eq nil
      execution_summary.title_en = 'New title'

      expect(execution_summary.title_en).to eq 'New title'
      expect(execution_summary.changes[:title_en]).to eq [nil, "New title"]

So, I had fr title value in title_en