Fix potential WTF in NotifictionRequest

Since should not be defaulted to Now since its really really unlikely you just got a notifiction during the time you created this message and the time GitHub received the message. If you don't change the `Since` you'll potentially always get 0 results. Instead, change the default behavior to MinValue so all Notifictions are returned - much like the default value of GetAllForCurrent without the NotificationsRequest argument.