Radio buttons stringify object values when clicked

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

Bug report

### What is the current behavior?

If a radio button's `value` property is an object, then clicking that radio button will cause the string `"[object Object]"` to be stored to the form values.

### What is the expected behavior?

The [FieldProps documentation](https://final-form.org/docs/react-final-form/types/FieldProps) states that the `value` property may be `any` type for checkboxes and radio buttons. I would expect that the object should be stored to the form without being converted to a string.

### Sandbox Link

https://codesandbox.io/s/react-final-form-radio-object-values-bug-c927j?file=/index.js

### What's your environment?

node: 14.16.1
react: 16.14.0
final-form: 4.20.2
react-final-form: 6.5.3
OS: macOS 10.14.6
Browser: Chrome 90.0.4430.212

### Other information

The initial value and value comparison works properly. The bug is only triggered when the selected radio button changes.