Description:
------------
If using the DatePeriod->interval the DateInterval object has not been correctly initialized by its constructor.

With this bug it is impossible to take a DatePeriod object as one argument in a function :(

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

$start    = new DateTime('2015-09-01');
$end      = new DateTime('2015-09-02');
$interval = new DateInterval('P1D');
$period   = new DatePeriod($start, $interval, $end); 

foreach ($period as $current) {
    $currentEnd = clone $current;
    $currentEnd->add($period->interval);
    var_dump($current, $currentEnd);
}

Expected result:
----------------
object(DateTime)#6 (3) {
    ["date"]=> string(26) "2015-09-01 00:00:00.000000"
    ["timezone_type"]=> int(3)
    ["timezone"]=> string(16) "Europe/Amsterdam"
}
object(DateTime)#7 (3) {
    ["date"]=> string(26) "2015-09-02 00:00:00.000000"
    ["timezone_type"]=> int(3)
    ["timezone"]=> string(16) "Europe/Amsterdam"
}


Actual result:
--------------
Warning: DateTime::add(): The DateInterval object has not been correctly initialized by its constructor in /in/tmZN9 on line 10
object(DateTime)#6 (3) {
    ["date"]=> string(26) "2015-09-01 00:00:00.000000"
    ["timezone_type"]=> int(3)
    ["timezone"]=> string(16) "Europe/Amsterdam"
}
object(DateTime)#7 (3) {
    ["date"]=> string(26) "2015-09-01 00:00:00.000000"
    ["timezone_type"]=> int(3)
    ["timezone"]=> string(16) "Europe/Amsterdam"
}