`draft` in `update` collection method should be writable even if collection type is readonly

[pawelblaszczyk5]

Hello 👋

As I understand the intention behind update collection method is to get mutable, proxied object, like immer. I think it that case the item passed to the update callback should be marked as mutable:

Currently my collection items are readonly (because that's reality, they're readonly by default). Would it be possible to mark them here as writable using some generic DeepWritable type? I'd be open to making a PR if that's something you're open to!

Thanks in advance!

[KyleAMathews]

Yeah sure, makes sense to make sure the type is writable!