SetCookie can't be constructed due to private field

[lnicola]

The inner field of SetCookie is private, so it can't be constructed by the users of the crate.

[kaj]

Some kind of semantic constructor would be nice, maybe using a builder pattern, to make it possible to do something like:

    Response::builder()
        .something()
        .typed_header(SetCookie::build("NAME")
            .same_site_strict()
            .http_only()
            .value("value"),
        )
        .body(something)

(The typed_header method on a response builder is suggested in #42)