CanonicalName should casefold non-US-ASCII chars

According to Section 6.2 [1] of RFC 4034,all uppercase US-ASCII letters in the
owner name of the RR are replaced by the corresponding lowercase US-ASCII
letters.

This updates CanonicalName to conform to the RFC.

[1] https://www.rfc-editor.org/rfc/rfc4034#section-6.2

Fixes miekg#1434

defaults.go

@@ -5,6 +5,7 @@ import (
 	"net"
 	"strconv"
 	"strings"
+	"unicode"
 )
 
 const hexDigit = "0123456789abcdef"
@@ -330,8 +331,18 @@ func Fqdn(s string) string {
 
 // CanonicalName returns the domain name in canonical form. A name in canonical
 // form is lowercase and fully qualified. See Section 6.2 in RFC 4034.
+// According to the RFC all uppercase US-ASCII letters in the owner name of the
+// RR areeplaced by the corresponding lowercase US-ASCII letters.
 func CanonicalName(s string) string {
-	return strings.ToLower(Fqdn(s))
+	var result strings.Builder
+	for _, ch := range s {
+		if unicode.IsUpper(ch) && (ch >= 0x00 && ch <= 0x7F) {
+			result.WriteRune(unicode.ToLower(ch))
+		} else {
+			result.WriteRune(ch)
+		}
+	}
+	return Fqdn(result.String())
 }
 
 // Copied from the official Go code.

labels_test.go

@@ -243,6 +243,8 @@ func TestCanonicalName(t *testing.T) {
 		"example.test":     "example.test.",
 		"Lower.CASE.test.": "lower.case.test.",
 		"*.Test":           "*.test.",
+		"ÉxamplE.com":      "Éxample.com.",
+		"É.com":            "É.com.",
 	} {
 		if got := CanonicalName(s); got != expect {
 			t.Errorf("CanonicalName(%q) = %q, expected %q", s, got, expect)