Can't use custom mutators with FieldArray

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

Potential bug report though it might be by design

### What is the current behavior?

If I create a custom mutator and then try to use it in a field array component by calling `fields.customMutator` then the values in the array passed into the custom mutator are overridden.

### What is the expected behavior?

I would expect that the parameters I pass into the function would not be overridden

### Sandbox Link

https://codesandbox.io/s/react-final-form-field-arrays-38gj3

### What's your environment?

"react-final-form": "6.3.0",
"react-final-form-arrays": "3.1.0",

### Other information

The mutator works fine when it is passed in directly but if that is how they're supposed to be used then why do they show up as properties of the fields parameter?