V3 arrayMutators map not working as expected

### What is the current behavior?
When using `map` over fields, there's no iteration on the values.

### What is the expected behavior?
Values are passed into map function.

### Sandbox Link
V3 (Broken): https://codesandbox.io/s/react-final-form-field-arrays-vnzwt
V2.0.3 (Working): https://codesandbox.io/s/react-final-form-field-arrays-h0wml

### What's your environment?
```
"final-form": "4.12.0",
"final-form-arrays": "1.1.2",
"react": "16.8.6",
"react-dom": "16.8.6",
"react-final-form": "5.1.1",
"react-final-form-arrays": "3.0.0",
```

### Other information
It could be that all other array mutations suffer too currently. Likewise, the `Add Customer` and `Remove Customer` functionality is broken.