made asc sorting optional and added regression tests

helper.go

@@ -1,7 +1,6 @@
 package filter
 
 import (
-	"errors"
 	"reflect"
 	"strconv"
 	"strings"
@@ -13,9 +12,12 @@ const (
 )
 
 func checkFields(s []string, val string) error {
+	if len(s) == 0 {
+		return newErr("invalid field: " + val)
+	}
 	for _, v := range s {
 		if v == "" {
-			return errors.New("invalid subfield: " + val)
+			return newErr("invalid subfield: " + val)
 		}
 	}
 	return nil