failing test for property delete in schema

[amir-arad]

This PR adds a failing test for when deleting a property from a schema object does not trigger change events.

[endel]

That's an interesting find @amir-arad, thanks for the test-case.

As a workaround, you can assign null or undefined to the value you want to delete. It would be nice to support delete state.field directly, though.

I think we'd need another proxy layer behind every Schema instance to allow this.

[amir-arad]

@endel sure thing. I'm just documenting what I find. been helping @a-rts write a mobx client, and found this edge, so it's yours to decide what to do with it.