Description:
------------
When initialising the DateTime constructor without specifying the time, the microseconds property isn't initialised.

Test script:
---------------
<?php
$a = new DateTimeImmutable;
var_dump($a);
?>

Expected result:
----------------
class DateTimeImmutable#1 (3) {
  public $date =>
  string(26) "2016-09-19 11:20:18.723145"
  public $timezone_type =>
  int(3)
  public $timezone =>
  string(13) "Europe/London"
}

Actual result:
--------------
class DateTimeImmutable#1 (3) {
  public $date =>
  string(26) "2016-09-19 11:20:18.000000"
  public $timezone_type =>
  int(3)
  public $timezone =>
  string(13) "Europe/London"
}