[GH-234] Criteria filtering doesn't work with DateTime instance

This issue is created automatically through a Github pull request on behalf of ludekstepan:

Url: https://github.com/doctrine/common/pull/234

Message:

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)
;
```