Possible fix for broken callbacks

[elon-gs]

Callbacks aren't working for me with 2.0.2. I believe the condition I've changed was to blame.

[ehaynes99]

Looks good. This bug is actually more sinister than just not calling them. It calls the callback n - 1 times, where n is the number of subscribers. If n === 1, it gets called zero times, but if n === 4, it gets called 3 times.

[jherencia]

I'm having the same problem and this commit fixed it.

@jamiebuilds Probably is a big bug, if you need some help to test it, i'll be happy to try.

[chip4]

Created a CodeSandbox demonstrating the issue: https://codesandbox.io/s/k9n8x5p447.
In the current release of unstated (2.0.3) setState callbacks are unusable. Given that a goal of this library (as I understand it from here: #1 (comment)) is to make it as seamless as possible to refactor component state into container state and that React's setState supports callbacks, I think this is an issue worth addressing.
What needs to happen to get this fix merged?

Thanks for all the hard work that goes into building and maintaining this!