fix(iban): fix luhn algorithm implementation

Closes #4

Author: peterlgh7

iban/national_checksum.go

@@ -336,5 +336,5 @@ func luhn(s string) int {
 		sum = (sum + n) % 10
 	}
 
-	return 10 - sum%10
+	return (10 - sum%10) % 10
 }

iban/parse_test.go

@@ -48,6 +48,7 @@ func TestParse(t *testing.T) {
 		{In: "EE44 3300 3384 0010 0007", Expect: "EE44 3300 3384 0010 0007"},
 		{In: "FI21 1234 5600 0007 85", Expect: "FI21 1234 5600 0007 85"},
 		{In: "FI14 1009 3000 1234 58", Expect: "FI14 1009 3000 1234 58"},
+		{In: "FI32 9360 7346 0370 70", Expect: "FI32 9360 7346 0370 70"},
 		{In: "FR14 2004 1010 0505 0001 3M02 606", Expect: "FR14 2004 1010 0505 0001 3M02 606"},
 		{In: "FR76 3000 6000 0112 3456 7890 189", Expect: "FR76 3000 6000 0112 3456 7890 189"},
 		{In: "HU42 1177 3016 1111 1018 0000 0000", Expect: "HU42 1177 3016 1111 1018 0000 0000"},