Remove a now unused parameter
Remove an unnecessary (and broken) optional parameter

We shouldn't under any circumstances be generating IDs for people that
are just slugified names any more; they should be integer IDs based on
the maximum previous person ID - it's picked in
create_person_with_id_retries now.
Add tests for the current behaviour of get_person_data_from_dict
Allow fields to be cleared by setting them to empty

This is another effect of PopIt's non-standard behaviour in response
to PUT requests at the moment. (See
https://github.com/mysociety/popit-api/issues/95 )

We can fix this by making sure that simple fields are set to null if
they're not specified, and clearing links and contact details by setting
them to [] in the "purging" PUT before the real one.  sigh.

Fixes #33