When using "push" and then "remove" the form stays dirty.

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

I believe this is a bug, but it is possible that I am not understanding something.

### What is the current behavior?

When using a `FieldArray`, `push`, and `remove` you can end up with a dirty form, when I don't think it should be dirty.

I forked the Simple Demo: https://codesandbox.io/s/32p402x3om
* Click `Add Customer`
* Click the `X` to remove the just added field.

The form stays dirty, but I would expect it to be back to pristine.

### What is the expected behavior?
* I think the form should not be dirty, since you have undone the adding of a field (similar to typing in a text input, and then deleting the text)

### Sandbox Link

https://codesandbox.io/s/32p402x3om

### What's your environment?

React Final form. Mac OSX Chrome.

### Other information

Thank you for a great library. Let me know if I can provide more information.