Description:
------------
DateTime::__construct should allow a DateTime object to be passed as parameter to emulate good object desing (Copy constructor).

This wouldn't break anything.

Reproduce code:
---------------
<?php
date_default_timezone_set('Europe/Helsinki');

class MyClass
{
	private $myDateTime;
	public function setTime($datetime = null) { $this->myDateTime = new DateTime($datetime); }
	public function getTime($format = "Y-m-d") { return $this->myDateTime->format($format); }
}

$class = new MyClass();
$class->setTime("7.8.2009");
var_dump($class->getTime());
$class->setTime(null);
var_dump($class->getTime());
$dt = new DateTime('15.5.2005');
$class->setTime($dt);
var_dump($class->getTime());
?>

Expected result:
----------------
string(10) "2009-08-07"
string(10) "2009-08-07"
string(10) "2005-05-15"

Actual result:
--------------
string(10) "2009-08-07"
string(10) "2009-08-07"
PHP Fatal error:  Uncaught exception 'Exception' with message 'DateTime::__construct() expects parameter 1 to be string, object gi
ven' in D:\temp\php\datetime.php:7
Stack trace:
#0 D:\temp\php\datetime.php(7): DateTime->__construct(Object(DateTime))
#1 D:\temp\php\datetime.php(17): MyClass->setTime(Object(DateTime))
#2 {main}
  thrown in D:\temp\php\datetime.php on line 7