Description:
------------
The following code will return `DateTimeImmutable` when using the `createFromFormat` method.


However, the following code will return `ChildDateTimeImmutable` when `setDate` is executed.

Test script:
---------------
<?php
class ChildDateTimeImmutable extends DateTimeImmutable
{
}

$child = new ChildDateTimeImmutable('2018-08-01');
assert($child->createFromFormat('Y-m-d', '2018-01-01') instanceof ChildDateTimeImmutable);
assert($child->setDate(2018, 1, 1) instanceof ChildDateTimeImmutable);

Expected result:
----------------
I expectation is to return `ChildDateTimeImmutable` when using` createFromFormat`.

Actual result:
--------------
Warning: assert(): assert($child->createFromFormat('Y-m-d', '2018-01-01') instanceof ChildDateTimeImmutable) failed in /private/tmp/20180824140235.php on line 7

Call Stack:
    0.0009     361408   1. {main}() /private/tmp/20180824140235.php:0
    0.0049     362504   2. assert() /private/tmp/20180824140235.php:7