Add Partial to setState

[pmkroeker]

Fixes some typing issues when using keyof State as a key in setState. Fixes #1778.

Allows for this:

interface IState {
  username: string;
  password: string;
}

function onInputChange(e: Event, field: keyof IState) {
  const target = e.target as HTMLInputElement;
  setState({ [field]: target.value });
}

Without throwing errors of missing fields.

[coveralls]

Coverage increased (+0.8%) to 99.688% when pulling dcd09de on pmkroeker:patch-1 into c399a24 on preactjs:master.

[marvinhagemeister]

Sweet 👍 💯 Thank you so much for the PR. Let's add a little test case to make sure were not running into this in the future again. Just declaring the component in the linked issue should do it 🎉

[JoviDeCroock]

The typings guru strikes again, great job!

[marvinhagemeister]

Sweet 👍 Thank you so much for the PR🙌💯