mime/quotedprintable: accept bytes >= 0x80

RFC 2045 doesn't permit non-ASCII bytes, but some systems send them
anyhow. With this change, we accept them. This does make it harder to
validate quotedprintable data, but on balance this seems like the best
approach given the existence of systems that generate invalid data.

Fixes #22597

Change-Id: I9f80f90a60b76ada2b5dea658b8dc8aace56cdbd
Reviewed-on: https://go-review.googlesource.com/121095Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/mime/quotedprintable/reader.go

@@ -123,6 +123,10 @@ func (r *Reader) Read(p []byte) (n int, err error) {
 			r.line = r.line[2:] // 2 of the 3; other 1 is done below
 		case b == '\t' || b == '\r' || b == '\n':
 			break
+		case b >= 0x80:
+			// As an extension to RFC 2045, we accept
+			// values >= 0x80 without complaint. Issue 22597.
+			break
 		case b < ' ' || b > '~':
 			return n, fmt.Errorf("quotedprintable: invalid unescaped byte 0x%02x in body", b)
 		}

src/mime/quotedprintable/reader_test.go

@@ -37,7 +37,7 @@ func TestReader(t *testing.T) {
 		{in: " A B =\n C ", want: " A B  C"}, // lax. treating LF as CRLF
 		{in: "foo=\nbar", want: "foobar"},
 		{in: "foo\x00bar", want: "foo", err: "quotedprintable: invalid unescaped byte 0x00 in body"},
-		{in: "foo bar\xff", want: "foo bar", err: "quotedprintable: invalid unescaped byte 0xff in body"},
+		{in: "foo bar\xff", want: "foo bar\xff"},
 
 		// Equal sign.
 		{in: "=3D30\n", want: "=30\n"},
@@ -65,6 +65,8 @@ func TestReader(t *testing.T) {
 		// Example from RFC 2045:
 		{in: "Now's the time =\n" + "for all folk to come=\n" + " to the aid of their country.",
 			want: "Now's the time for all folk to come to the aid of their country."},
+		{in: "accept UTF-8 right quotation mark: ’",
+			want: "accept UTF-8 right quotation mark: ’"},
 	}
 	for _, tt := range tests {
 		var buf bytes.Buffer