Bug 21465: Don't throw duplicate userid error if userid belongs to the matched patron

Bug 21465: Don't throw duplicate userid error if userid belongs to the matched patron

To test:
 1 - Export your patrons
    a - Create a report 'SELECT * FROM borrowers'
    b - Run and save the report as csv (check your delimiter)
    c - Delete the borrowernumebr column
 2 - Use the Patron Import tool to import the csv from above
 3 - Set matching to 'cardnumber'
 4 - Set 'If matching record is already in the borrowers table:' to
Overwrite
 5 - Import
 6 - None are import because of matchign userid (their own)
 7 - Apply patch
 8 - Repeat
 9 - Patrons are successfully overwritten
10 - prove -v t/db_dependent/Koha/Patrons/Import.t
11 - prove -v t/db_dependent/Koha/Patrons.t

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>