Description:
------------
If we create a custom class that inherits from DateTimeImmutable, this class can be given as parameter to DatePeriod.

However, when iterating or trying to get the start or end dates, DatePeriod will try to return instances of our custom class but without using its constructor, which will produce incoherent objects.

See this PR for more details: https://github.com/thecodingmachine/safe/pull/227/files

Test script:
---------------
<?php

class CustomDateTimeImmutable extends \DateTimeImmutable {

    private $foo;

    public function __construct($time = "now", $timezone = NULL)
    {
        parent::__construct($time, $timezone);
        $this->foo = "foo"; 
    }
    
    public function getFoo()
    {
        return $this->foo;
    }

}

$datePeriod = new \DatePeriod(new CustomDateTimeImmutable('2020-01-01'), new \DateInterval('P1D'), (new CustomDateTimeImmutable('2020-01-03'))->modify('+1 day'));

var_dump($datePeriod->getStartDate()->getFoo());

$strings = [];
foreach ($datePeriod as $date) {
    var_dump($date->getFoo());
}



Expected result:
----------------
"foo"
"foo"
"foo"
"foo"

Actual result:
--------------
NULL
NULL
NULL
NULL