FormState generic type requires 1 type argument.

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

Bug report

### What is the current behavior?

My application has some code like this:
```js
onChange?: (formState: FormState) => void;
```

So, after `FormState` definition was updated to have `FormValues` generic type for `values`, I have this type of error: `Generic type 'FormState' requires 1 type argument(s).`

### What is the expected behavior?

Make generic for `FormState` optional and set default value to `AnyObject ` as it was before update.

```js
export interface FormState {
```

### What's your environment?

```json
{
"react": "^16.8.6",
"react-dom": "^16.8.4",
"react-final-form": "^6.1.0",
"final-form": "^4.14.1",
}
```