Add @property setters for Cookie properties.

It's kinda weird to have idiomatic D @property getters for members of class Cookie like isSecure, path, maxAge, etc, but then for *setting* them only offer Java-like setSecure, setPath, setMaxAge, etc.

I realize the set*() funcs are to allow chaining, and that's fine, but I'm not sure that should preclude inclusion of idiomatic setters.