Fix for utf8spn not matching ASCII characters

[roxas232]

This example shows why the previous test values were off by one:

#include <iostream>
#include <string.h>

using namespace std;

int main()
{
    auto len = strspn("abcdefg", "abc");
    cout<< len << endl; // 3

    len = strspn("abcdefg", "abcdefg");
    cout<< len; // 7

    return 0;
}