Custom Input buttons for radio-like input

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

Maybe a bug, but probably ignorance of custom input usage.

Basically, I'm looking for the best way to do [this](https://codesandbox.io/s/ojx3wyyjp6) and it's not clear to me in the docs...

### What is the current behavior?

```jsx
true}>
{({input, meta}) => (



)}

yields `Warning: Can't call setState on a component that is not yet mounted.` on first and only first click
```

### What is the expected behavior?

no React lifecycle warning

### Sandbox Link

[Stack](https://stackoverflow.com/questions/52864734/how-can-i-create-custom-fields-in-react-final-form)

[Codesandbox](https://codesandbox.io/s/ojx3wyyjp6)

### What's your environment?

"react-final-form": "^3.6.7",
"final-form": "^4.10.0",
Chrome

### Other information

Best I can tell, this is happening in `registerField` call in final-form repo. The following line:

```js
change: function change(value) {
return api.change(name, value);
},
```