Checkbox validation only after focus change

### Are you submitting a **bug report** or a **feature request**?

Maybe it's a feature but it seems more like a bug.

### What is the current behavior?


If I click the checkbox (which is required) and uncheck it, the validation only takes place once I click somewhere else.

### What is the expected behavior?

I would expect the validation to happen as soon as I uncheck the box.

### Sandbox Link

Please have a look at the example in [codesandbox](https://codesandbox.io/s/react-final-form-field-level-validation-example-vvl4w?fontsize=14)

### What's your environment?

"react-final-form": "^6.3.0",
"final-form": "^4.16.1",