datetime isoformat inconsistency #29

When making a booking via practable-python, the returned format of the booking date and time is different (YYYY-MM-DDTHH:MM:SS.sssZ) from that when a booking is made via the booking UI (YYYY-MM-DD HH:MM:SS.ssssss+00:00).

The source of this is likely a difference in the format of the booking datetime being sent to the booking server i.e. a difference between python and javascript.