Description:
------------
`DateTimeImmutable::createFromFormat()` does not appear to allow timestamps in Atom (RFC 3339) format to have a decimal point for time units. RFC 3339 derives the relevant specification from ISO-8601, which states:

> A decimal mark, either a comma or a dot [...] is used as a separator between the time element and its fraction

Alas, when trying to parse such a timestamp, `createFromFormat()` returns `false`, which is a bug of its own (https://bugs.php.net/bug.php?id=64923).

See it in action here: https://3v4l.org/OSfKR

Interestingly, when creating a date from a timestamp without the decimal part, the resulting instance has the `$date` property set to `2019-10-10 15:32:38.000000`, which does in fact contain the decimal part. This probably means that internally, PHP does cater for fractions.

Test script:
---------------
$dateString = '2019-10-10T15:32:38.916Z';
$date = \DateTimeImmutable::createFromFormat(DATE_ATOM, $dateString);

if (!($date instanceof \DateTimeInterface)) {
    throw new RangeException('Looks like PHP cannot parse decimals in time segment');
}

Expected result:
----------------
No exception is thrown.

Actual result:
--------------
An exception is thrown, indicating that a date instance could not be produced from the timestamp.