MONGOID-5104 added *_previously_changed and *_previously_changed? helpers.

Rails also supports a *_previously_was helper: https://www.bigbinary.com/blog/rails-6-1-adds-_previously_was-attribute-methods

Please add this to Mongoid; I can't upgrade my app without it.

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On a side note, MONGOID-5104 is a big and dangerous change in my app. While I agree with the change, it would have been nice to have a global config toggle ("Use previously was") or even a model-level toggle that would facilitate upgrading.