Fix destroy so browser-side cookie session data is reset

The current release deletes the cookie so it isn't sent, but doesn't erase it browser side so the session reappears on the next request.  This pull request has a fix to ensure browser-side state is destroyed.

It also cleans up, slightly, how the cookie is written.  Unfortunately, Dancer::Session::Abstract isn't sufficiently well factored so I had to copy code in from there and tweak it.  Oh, well.