Description:
------------
array_unique() returns duplicates for array which contains objects with DateTime 
instances.

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

class Foo
{
	public $dt;

	function __construct($dt)
	{
		$this->dt = $dt;
	}
}

$foo = new Foo(new DateTime);
$std = new stdClass;
$arr = [$foo, $foo, $std, $std, $std];

var_dump(array_unique($arr, SORT_REGULAR));

Expected result:
----------------
array(4) {
  [0]=>
  object(Foo)#1 (1) {
    ["dt"]=>
    object(DateTime)#2 (3) {
      ["date"]=>
      string(19) "2012-08-30 15:18:01"
      ["timezone_type"]=>
      int(3)
      ["timezone"]=>
      string(13) "Europe/Prague"
    }
  }
  [2]=>
  object(stdClass)#3 (0) {
  }
}

Actual result:
--------------
array(4) {
  [0]=>
  object(Foo)#1 (1) {
    ["dt"]=>
    object(DateTime)#2 (3) {
      ["date"]=>
      string(19) "2012-08-30 15:18:01"
      ["timezone_type"]=>
      int(3)
      ["timezone"]=>
      string(13) "Europe/Prague"
    }
  }
  [1]=>
  object(Foo)#1 (1) {
    ["dt"]=>
    object(DateTime)#2 (3) {
      ["date"]=>
      string(19) "2012-08-30 15:18:01"
      ["timezone_type"]=>
      int(3)
      ["timezone"]=>
      string(13) "Europe/Prague"
    }
  }
  [2]=>
  object(stdClass)#3 (0) {
  }
  [4]=>
  object(stdClass)#3 (0) {
  }
}