Allow triggering a hook when ANY field changes

[WhyNotHugo]

This small change allows triggering a hook when any field for a model changes.

Unlike when_any, this doesn't require enumerating all fields, which can be a pain for very large model, and it can be very easy to forget about fields when adding them to the model.

Docs and tests should clarify any final details around this.

[WhyNotHugo]

Hang on, tests for this are wrong, need to rework them.

[emiel]

@rsinger86: This PR is a quite outdated now but I was curious if the change is still on the table. Can we breathe new life into this or would it be dead-on-arrival. ;)

[WhyNotHugo]

I'm no longer working on the project where we used this, so I don't think I'll be picking it up any time soon. Feel free to adopt this PR if you're interested.