net/mail: allow us-ascii encoding

Fixes #6611.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/14990045

src/net/mail/message.go

@@ -28,6 +28,7 @@ import (
 	"strconv"
 	"strings"
 	"time"
+	"unicode"
 )
 
 var debug = debugT(false)
@@ -445,7 +446,7 @@ func decodeRFC2047Word(s string) (string, error) {
 		return "", errors.New("address not RFC 2047 encoded")
 	}
 	charset, enc := strings.ToLower(fields[1]), strings.ToLower(fields[2])
-	if charset != "iso-8859-1" && charset != "utf-8" {
+	if charset != "us-ascii" && charset != "iso-8859-1" && charset != "utf-8" {
 		return "", fmt.Errorf("charset not supported: %q", charset)
 	}
 
@@ -466,6 +467,16 @@ func decodeRFC2047Word(s string) (string, error) {
 	}
 
 	switch charset {
+	case "us-ascii":
+		b := new(bytes.Buffer)
+		for _, c := range dec {
+			if c >= 0x80 {
+				b.WriteRune(unicode.ReplacementChar)
+			} else {
+				b.WriteRune(rune(c))
+			}
+		}
+		return b.String(), nil
 	case "iso-8859-1":
 		b := new(bytes.Buffer)
 		for _, c := range dec {

src/net/mail/message_test.go

@@ -194,6 +194,16 @@ func TestAddressParsing(t *testing.T) {
 				},
 			},
 		},
+		// RFC 2047 "Q"-encoded US-ASCII address. Dumb but legal.
+		{
+			`=?us-ascii?q?J=6Frg_Doe?= <joerg@example.com>`,
+			[]*Address{
+				{
+					Name:    `Jorg Doe`,
+					Address: "joerg@example.com",
+				},
+			},
+		},
 		// RFC 2047 "Q"-encoded UTF-8 address.
 		{
 			`=?utf-8?q?J=C3=B6rg_Doe?= <joerg@example.com>`,