[SimpleFormIterator] Support default fields value when click add button

[scq000]

I have the following code:

{
  id: 123
  backlinks: [
        {
            date: '2012-08-10T00:00:00.000Z',
            url: 'http://example.com/foo/bar.html',
        },
        {
            date: '2012-08-14T00:00:00.000Z',
            url: 'https://blog.johndoe.com/2012/08/12/foobar.html',
        }
   ]
}

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

<ArrayInput source="backlinks">
    <SimpleFormIterator>
        <DateInput source="date" />
        <TextInput source="url" />
    </SimpleFormIterator>
</ArrayInput>

When I click add button, a new record will be rendered in the Backlinks field, but the default value is undefined. After clicking the add button, the data structure will be like this:

{
  id: 123
  backlinks: [
        {
            date: '2012-08-10T00:00:00.000Z',
            url: 'http://example.com/foo/bar.html',
        },
        {
            date: '2012-08-14T00:00:00.000Z',
            url: 'https://blog.johndoe.com/2012/08/12/foobar.html',
        },
       {}
   ]
}

But I would like to set the default value when I click the add button.

[djhi]

Duplicate of #2489.
Also, please follow the issue template