This patch adds middle_name to the accaeptable fields in API requests
and responses.
Test plan
1) Search for a user using the API
2) Confirm the API responds with a 200
Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>