Motivation:
DefaultCookie currently used an undocumented magic value for undefined
maxAge.
Clients need to be able to identify such value so they can implement a
proper CookieJar.
Ideally, we should add a `Cookie::isMaxAgeDefined` method but I guess
we can’t add a new method without breaking API :(
Modifications:
Add a new constant on `Cookie` interface so clients can use it to
compare with value return by `Cookie.maxAge` and decide if `maxAge` was
actually defined.
Result:
Clients have a better documented way to check if the maxAge attribute
was defined.