Detach (DateTime) objects in original object data

The PR makes Doctrine ORM update the database when you execute something like this:

```php
// MyEntity.php

/**
 * @Entity
 */
class MyEntity
{
    // ...

    /**
     * @Column(type="datetime")
     */
     private $created;

    // ...

    public function setCreated(\DateTime $created)
    {
        $this->created = $created;
    }

    public function getCreated()
    {
        return $this->created;
    }

    // ...
}
```

```php
// Current problem

$entity = $entityManager->find('MyEntity', 1);

$entity->getCreated()->setDate('2001-01-01');

$entityManager->persist($entity);

$entityManager->flush();

// At this point no changes have been made to the database!
```

The reason this doesn't work currently is because the array of original data stored in the UnitOfWork contains the actual DateTime object which is in the entity. When you update the object in the entity the original data is updated also so the entity are the original value are still equal and therefore the object is not seen as changed.

I have chosen to clone all objects rather than just DateTime object include more types are included in the future.