net/mail: correctly handle whitespaces when formatting an email address

Whitespace characters are allowed in quoted-string according to RFC 5322 without
being "Q"-encoding. Address.String() already always formats the name portion in
quoted string, so whitespace characters should be allowed in there.

Fixes #6641.

LGTM=dave, dsymonds
R=golang-codereviews, gobot, dsymonds, dave
CC=golang-codereviews
https://golang.org/cl/55770043

src/pkg/net/mail/message.go

@@ -159,7 +159,9 @@ func (a *Address) String() string {
 	// If every character is printable ASCII, quoting is simple.
 	allPrintable := true
 	for i := 0; i < len(a.Name); i++ {
-		if !isVchar(a.Name[i]) {
+		// isWSP here should actually be isFWS,
+		// but we don't support folding yet.
+		if !isVchar(a.Name[i]) && !isWSP(a.Name[i]) {
 			allPrintable = false
 			break
 		}
@@ -167,7 +169,7 @@ func (a *Address) String() string {
 	if allPrintable {
 		b := bytes.NewBufferString(`"`)
 		for i := 0; i < len(a.Name); i++ {
-			if !isQtext(a.Name[i]) {
+			if !isQtext(a.Name[i]) && !isWSP(a.Name[i]) {
 				b.WriteByte('\\')
 			}
 			b.WriteByte(a.Name[i])
@@ -535,3 +537,9 @@ func isVchar(c byte) bool {
 	// Visible (printing) characters.
 	return '!' <= c && c <= '~'
 }
+
+// isWSP returns true if c is a WSP (white space).
+// WSP is a space or horizontal tab (RFC5234 Appendix B).
+func isWSP(c byte) bool {
+	return c == ' ' || c == '\t'
+}

src/pkg/net/mail/message_test.go

@@ -277,6 +277,14 @@ func TestAddressFormatting(t *testing.T) {
 			&Address{Name: "Böb", Address: "bob@example.com"},
 			`=?utf-8?q?B=C3=B6b?= <bob@example.com>`,
 		},
+		{
+			&Address{Name: "Bob Jane", Address: "bob@example.com"},
+			`"Bob Jane" <bob@example.com>`,
+		},
+		{
+			&Address{Name: "Böb Jacöb", Address: "bob@example.com"},
+			`=?utf-8?q?B=C3=B6b_Jac=C3=B6b?= <bob@example.com>`,
+		},
 	}
 	for _, test := range tests {
 		s := test.addr.String()