Description:
------------
The combined assignment operators do not work for DateTime::$f

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

$buggy = new DateInterval('P0Y');
$buggy->f += 0.01;

$ok = new DateInterval('P0Y');
$ok->f = $ok->f + 0.01;

var_dump($buggy->f);
var_dump($ok->f);


Expected result:
----------------
/tmp/mutime.php:9:
double(0.01)
/tmp/mutime.php:10:
double(0.01)


Actual result:
--------------
PHP Notice:  Undefined property: DateInterval::$f in /tmp/mutime.php on line 4
PHP Stack trace:
PHP   1. {main}() /tmp/mutime.php:0
/tmp/mutime.php:9:
double(0)
/tmp/mutime.php:10:
double(0.01)