Description:
------------
A DateTime object with timezone_type=1 is always set to timezone=+00:00 after the use of diff

Test script:
---------------
date_default_timezone_set( "Europe/Rome" );

$da = new DateTime('2010-06-22T00:00:00+02:00');
$a = new DateTime('2010/06/23');

print_r($da);

$da->diff($a);
// $da has been changed
print_r($da);

Expected result:
----------------
DateTime Object
(
    [date] => 2010-06-22 00:00:00
    [timezone_type] => 1
    [timezone] => +02:00
)
DateTime Object
(
    [date] => 2010-06-21 22:00:00
    [timezone_type] => 1
    [timezone] => +02:00
)


Actual result:
--------------
DateTime Object
(
    [date] => 2010-06-22 00:00:00
    [timezone_type] => 1
    [timezone] => +02:00
)
DateTime Object
(
    [date] => 2010-06-21 22:00:00
    [timezone_type] => 1
    [timezone] => +00:00
)