Form doesn't change pristine value after removing second item

### Are you submitting a **bug report** or a **feature request**?
Bug report
### What is the current behavior?
When initial values are set with minimum 2 entries, clicking remove on second item doesn't change pristine state of form

### What is the expected behavior?
Form shouldn't be pristine, allowing me to save changes
### Sandbox Link
Taken from readme simple example and added only following part
```
initialValues={{
customers: [
{
firstName: "John",
lastName: "Doe"
},
{
firstName: "Henry",
lastName: "Ford"
}
]
}}
```
https://codesandbox.io/s/react-final-form-field-arrays-eutnd

### 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": "4.1.0",
"react-final-form-arrays": "2.0.3",
```
Tested on Win10 Chrome

### Other information