Zend incompatibility: DateTime timezone incorrectly set

The timezone parameter of the `DateTime` constructor should only be used as a hint. Currently in HHVM it is stored as the actual timezone.

``` php
$tz = new DateTimeZone('Antarctica/South_Pole');
$time = new DateTime('@1301574225', $tz);
var_dump($time->getTimeZone()->getName());
```

Zend:
`string(6) "+00:00"`

HHVM:
`string(21) "Antarctica/South_Pole"`