Use isAscii...()

Elwig/Helpers/Utils.cs

@@ -131,7 +131,7 @@ namespace Elwig.Helpers {
         }
 
         public static int Modulo(string a, int b) {
-            if (a.Length == 0 || !a.All(char.IsDigit)) {
+            if (a.Length == 0 || !a.All(char.IsAsciiDigit)) {
                 throw new ArgumentException("First argument has to be a decimal string");
             } else if (b < 2) {
                 throw new ArgumentException("Second argument has to be greater than 1");