Description:
------------
DateInterval::format method just returns the parameter given.

Reproduce code:
---------------
<?php

$date1 = new DateTime('2009-06-19 20:00:00');
$date2 = new DateTime('2009-06-19 20:00:40');

$diff = $date1->diff($date2,TRUE);

print_r($diff);

var_dump($diff->format('H:i:s'));
var_dump($diff->format('Y-m-d H:i:s'));

?>


Expected result:
----------------
DateInterval Object
(
    [y] => 0
    [m] => 0
    [d] => 0
    [h] => 0
    [i] => 0
    [s] => 40
    [invert] => 0
    [days] => 0
)
string(5) "00:00:40"
string(11) "0000-00-00 00:00:40"



Actual result:
--------------
DateInterval Object
(
    [y] => 0
    [m] => 0
    [d] => 0
    [h] => 0
    [i] => 0
    [s] => 40
    [invert] => 0
    [days] => 0
)
string(5) "H:i:s"
string(11) "Y-m-d H:i:s"