Criteria filtering doesn't work with DateTime instance

Filtering associations doesn't work with DateTime instance as a comparison value because filtering uses === operator. It works with SQL backed filtering, but not on PHP collection level.

```php
$criteria = Criteria::create()
    ->where(Criteria::expr()->eq("birthday", new \DateTime("1982-02-17")))
    ->orderBy(array("username" => "ASC"))
    ->setFirstResult(0)
    ->setMaxResults(20)
;
```