/*      list_fields(table, field=nil)        */
static VALUE list_fields(int argc, VALUE* argv, VALUE obj)
{
    VALUE table, field;
    MYSQL* m = GetHandler(obj);
    MYSQL_RES* res;
    rb_scan_args(argc, argv, "11", &table, &field);
    res = mysql_list_fields(m, StringValuePtr(table), NILorSTRING(field));
    if (res == NULL)
        mysql_raise(m);
    return mysqlres2obj(res);
}