Fix KidnummerValidator incorrectly accepting invalid KID numbers

[leifthorbjornsen]

Fixes a bug in KidnummerValidator.isValid() where some invalid KID numbers were accepted.

Examples:

• 100030
• 100031
• 100033

The issue is in the mod11 check:

if ("-".equals(kMod11) || Integer.parseInt(kMod11) == k.getChecksumDigit()) {
    return; // valid
}

When the mod11 remainder is 1, there is no valid single digit that can serve as a checksum digit, so it returns - instead. The bug was that the validator treated this as an automatic pass, without checking that the KID number actually ended with -. As a result, any KID that failed mod10 and happened to produce a mod11 remainder of 1 would be incorrectly accepted.

The fix is to compare the calculated mod11 result directly with the last character of the KID:

String lastChar = kidnummer.substring(kidnummer.length() - 1);
if (kMod11.equals(lastChar)) {
    return; // valid
}

This works for both digits and - without special handling.
1000005- still passes because mod11 returns - and the last character is -, while 100030 now correctly fails because mod11 returns - but the last character is 0.

Also added a test for the bug case:

private static final String KIDNUMMER_INVALID_CHECKSUM_MOD10_AND_MOD11 = "100030";

@Test
public void testInvalidKidnummerFailingBothMod10AndMod11() {
    assertFalse(KidnummerValidator.isValid(KIDNUMMER_INVALID_CHECKSUM_MOD10_AND_MOD11));
}