Fix SimpleFormIterator for non empty array inputs

[NikitaVlaznev]

Issue

A warning

Each child in a list should have a unique "key" prop

inside CssTransition shows if a record with none empty array input was loaded, and the animation of adding a new element is broken.

How to reproduce

It can be reproduced using any resource with ArrayInput and SimpleFormIterator:

1. Open Edit for some record
2. Add few items to an array input
3. Save
4. Open Edit for that record again and see a warning
5. Add few more elements to the array input and see broken animation

For example you can add this input to the CategoryEdit form in the demo app:

import { ArrayInput, SimpleFormIterator } from 'react-admin';
...

<ArrayInput label="Test array" source="test">
    <SimpleFormIterator>
        <NumberInput
            source="weight"
            label="Weight"
            defaultValue={1}
        />
    </SimpleFormIterator>
</ArrayInput>

Reason

nextId is a ref, it is an object like {current: value}.
(nextId > 0) will always be false because ({} > 0) is always false.

Fix

Seems like .current is forgotten there.
With this fix there is no warnings and animation works fine.

[djhi]

Thanks!

[NikitaVlaznev]

@djhi, @fzaninotto, it would be great to see a hotfix release, because currently ArrayInputs look like broken. It is critical for those who use them with the current version.