Added support for using DateTime objects in cookies

As previously discussed: https://github.com/chriso/klein.php/pull/213#issuecomment-49532907
Absolutely, the ResponseCookieDataCollection implements all of those interfaces, making the interface **similar** to an array, but the field itself never contains an array.

Your IDE is struggling with the magic iteration in the foreach, but that's understandable given the dynamic nature of the language. A `foreach()` is an iteration construct in PHP, not an array iteration construct.

Regardless of the hint that your IDE might want to give it, documenting a field as an incorrect type is invalid.