Description:
------------
calling getTimestamp() on an instance of a DateTimeImmutable modify it (and return the modified date timestamp).

calling format('U') does not modify the date

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

$date = new DateTimeImmutable('2014-05');

$date2 = $date->modify('first day of next month midnight - 1 second');

var_dump($date2->format('c'));
var_dump($date2->format('U'));
var_dump($date2->format('c'));
var_dump($date2->getTimestamp());
var_dump($date2->format('U'));
var_dump($date2->format('c'));


Expected result:
----------------
string(25) "2014-05-31T23:59:59+00:00"
string(10) "1401580799"
string(25) "2014-05-31T23:59:59+00:00"
int(1401580799)
string(10) "1401580799"
string(25) "2014-05-31T23:59:59+00:00"


Actual result:
--------------
string(25) "2014-05-31T23:59:59+00:00"
string(10) "1401580799"
string(25) "2014-05-31T23:59:59+00:00"
int(1398988799)
string(10) "1398988799"
string(25) "2014-05-01T23:59:59+00:00"