fix: delete root keys with immer middleware

[simlmx]

Related Issues or Discussions

Fixes #2480

Summary

Currently when calling setState with the immer middleware, replace=false is assumed by default. This means that the old state is merged with immer's produce output. I don't think there is any reason to merge those states: immer's produce already returns a smart version of the whole state where copies are used whenever possible. The merging is mostly equivalent to replace=True, except when keys are deleted: the old deleted keys get merged with the new state.

I'm also assuming that when using the immer middleware, we always want to do something like

store.setState((state) => { state.x = 3})

and never

store.setState({x: 3})

so I simplified the middleware consequently.

I added a test to reproduce the bug and some other simple tests.

Someone with more experience with the immer middleware should double-check all this! There could very well be usage patterns that I'm not aware of.

Check List

• [x] yarn run prettier for formatting code and docs

@dai-shi @dbritto-dev Let me know if I can improve anything in the PR.