immerjs / immer / 28671913401

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fix: undefined assigned to a prototype-inherited key gets dropped (#1262)

If an object's prototype has a key set to undefined and you assign
undefined to that same key on a draft, immer treats it as a no-op
since `prop in state.copy_` walks the prototype chain and finds it
"already there." No own property gets created, no patch gets
recorded. Swapped that for the has() own-property check that's
already used a few lines up for the same purpose, and added a
regressions test.

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