Bug 20367: Avoid resetting userid when BorrowerUnwantedField contains userid

Bug 20367: Avoid resetting userid when BorrowerUnwantedField contains userid

To test:
1. In staff client, set your username to firstname
2. Add userid to BorrowerUnwantedField system preference
3. Go to your patron modification screen (memberentry.pl) and click Save
4. Observe you get kicked out into login screen, saying:
Error: You do not have permission to access this page.
Log in as a different user

5. Apply patch and restart plack

6. Set your username back to firstname
7. Repeat step 3
8. Observe you were not kicked out and your userid stays the same

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>