Type fix in ORM\PersistentCollection

The ```ORM\PersistentCollection``` class accepts anything as 3rd argument to its constructor while it should be either array or ```Collection``` object. The ```$this->coll``` phpdoc says that it's a ```Collection``` while the constructor's phpdoc says it's an array.

The class assumes ```Collection``` but there isn't any type check at all. I have added conversion of array into ```ArrayCollection``` in the constructor as in some (very unclear) cases it likes to die with a fatal error:

```Fatal error: Call to a member function add() on a non-object in Doctrine/ORM/PersistentCollection.php on line 169```