mime: Export RFC 2047 code

Fixes #4943
Fixes #4687
Fixes #7079

Change-Id: Ia96f07d650a3af935cd75fd7e3253f4af2977429
Reviewed-on: https://go-review.googlesource.com/7890
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/mail/message.go

@@ -20,9 +20,9 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
-	"internal/mime"
 	"io"
 	"log"
+	"mime"
 	"net/textproto"
 	"strings"
 	"time"
@@ -177,7 +177,7 @@ func (a *Address) String() string {
 		return b.String()
 	}
 
-	return mime.EncodeWord(a.Name) + " " + s
+	return mime.QEncoding.Encode("utf-8", a.Name) + " " + s
 }
 
 type addrParser []byte
@@ -333,9 +333,8 @@ func (p *addrParser) consumePhrase() (phrase string, err error) {
 			word, err = p.consumeAtom(true)
 		}
 
-		// RFC 2047 encoded-word starts with =?, ends with ?=, and has two other ?s.
-		if err == nil && strings.HasPrefix(word, "=?") && strings.HasSuffix(word, "?=") && strings.Count(word, "?") == 4 {
-			word, err = mime.DecodeWord(word)
+		if err == nil {
+			word, err = decodeRFC2047Word(word)
 		}
 
 		if err != nil {
@@ -423,6 +422,32 @@ func (p *addrParser) len() int {
 	return len(*p)
 }
 
+func decodeRFC2047Word(s string) (string, error) {
+	dec, err := rfc2047Decoder.Decode(s)
+	if err == nil {
+		return dec, nil
+	}
+
+	if _, ok := err.(charsetError); ok {
+		return s, err
+	}
+
+	// Ignore invalid RFC 2047 encoded-word errors.
+	return s, nil
+}
+
+var rfc2047Decoder = mime.WordDecoder{
+	CharsetReader: func(charset string, input io.Reader) (io.Reader, error) {
+		return nil, charsetError(charset)
+	},
+}
+
+type charsetError string
+
+func (e charsetError) Error() string {
+	return fmt.Sprintf("charset not supported: %q", string(e))
+}
+
 var atextChars = []byte("ABCDEFGHIJKLMNOPQRSTUVWXYZ" +
 	"abcdefghijklmnopqrstuvwxyz" +
 	"0123456789" +