arrays get reset with pristine values on change

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

Bug report

### What is the current behavior?

I have a form with the following characteristics:
My initial values are as follows:
```
{
apis: []
}
```
```
(


{render(renderProps)}


)}
/>
```

```

{({ fields }) => (


Select an API


Select an API

{Object.values(apiConstants)
.filter(
ky => !values.apis.find(el => el.api === ky.value)
)
.map(el => (

{el.key}

))}






{fields.map((name, index) => (








))}




)}

```
When I select an item in the Select component a `state` change occurs and the entire form resets with `keepDirtyOnReinitialize=false` or it keeps the changes with `keepDirtyOnReinitialize=true` but after adding a first `api`, the second `state` change causes the `api[0]` to be set to `undefined`.

### What is the expected behavior?

It should not reset form state on a state change.

### What's your environment?
```
"node": "8.12.0",
"react": "16.6.0",
"final-form": "4.11.0",
"final-form-arrays": "1.1.1",
"react-final-form": "4.0.2",
"react-final-form-arrays": "2.0.1",
```

Start by reproducing the provided Form and FieldArray example with the listed final-form, react-final-form, and array package versions. Trace what happens to the array when the Select causes a state change, and consider the issue done when existing array entries remain intact instead of being reset or becoming undefined.

Written by the indexing model from the issue text.