Minor issue(103) quick fix and some code clarifications

Fix issue 103 ``Pimple::factories`` may keep dead objects after ``extend()`` and ``offsetUnset()``

Other than that, 2 minor changes

 - in ``Pimple::raw()`` changed ``array_key_exists($id,$this->values)`` to ``isset($this->keys[$id])``. Isset will work much better/faster and, besides, any value set **must have** an identifying key in ``keys``

 - some minor code disentaglements ``return $this[$id] = $someValue;`` makes an implicit call to ``offsetSet()``, so I made it explicit