tokenHasExpired() not always return true/false

[azranel]

So this is small stuff and it doesn't change any way the library works but during using this awesome library I thought that tokenHasExpired() always returns true or false but when there is no expiration set in headers it returns null. This not that important because null is falsey value but I think it would be better to restrict it only to true and false value (similar as ? methods in Ruby should be written). If you think this can be improved then I will be more than happy to do PR. :)