Don't automatically add Expires in res.cookie if expires option is explicitly set to 0

[nbkhope]

This addresses #5150

Those who wish to exclude Expires (but still keep Max-Age from the final Set-Cookie statement cannot do so if maxAge option is passed to res.cookie).

This proposed solution allows one to exclude Expires if expires: 0 option is passed to res.cookie.

[tjarbo]

Allowing opts.expires to be of type number breaks the consistency, that in all other cases expires is of type Date. This is also enfored by the TS' type defintion @types/express as defined here.

Based on my comment on #5150 (comment), I would propose to remove opts.expires = new Date(Date.now() + maxAge) entirely. Developers can still set this value as part of the function call. The maintainers will need to decide if this would be a breaking-change.

[nbkhope]

@tjarbo thank you your feedback. I put zero because the docs said:

expires | Date | Expiry date of the cookie in GMT. If not specified or set to 0, creates a session cookie.

Ref https://github.com/expressjs/expressjs.com/blob/867a10d748c4f5c83dfdbc634e210621aa1a34a9/_includes/api/en/4x/res-cookie.md