Bug 15146: Format date of birth on patrons requesting modification page

Bug 15146: Format date of birth on patrons requesting modification page

To reproduce / test

- Log in to Opac
- Go to "your personal details"
- Request a change of the date of birth
- Log in to Staff client
- Follow link to Patrons requesting modifications
  (/koha/members/members-update.pl)
=> The date is not formatted in syspref format

- Apply patch, reload members-update.pl
=> The date is formatted in syspref format

- Approve changes
- Verify that changes are saved

This is independent from C4::Dates removal.

Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
Works as advertised. Changes saved Ok, Update patron record follows
dateformat syspref

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>