Added support for using DateTime objects in cookies

As previously discussed: https://github.com/chriso/klein.php/pull/213#issuecomment-49532907
Every formatting, variable casing style, and non-behavioral change in these tests should be reverted until a style is agreed upon. Even then, the changes should be in a separate PR.

As is, these changes make it very difficult to see what you've actually changed in the logic of the tests and produces changes to the tests that don't belong with the scope of this PR.