Setting array on ViewModel prune Array prototype.

[jacquarg]

 @set 'arrayField', ["a", "b"]
 console.log JSON.stringify @attributes.arrayField
> { "1": "a", "2": "b" } # expected : [ "a", "b"]

See https://github.com/jacquarg/cozy-contacts/blob/duplicates/client/app/views/models/merge.coffee#L21-L22

Setting directly @attributes.arrayField = ["a", "b"] works.