mime/quotedprintable: accept LWSP-char after =

SP and HTAB are allowed after a = before the following CRLF.

RFC 2045 section 6.7 describes the ABNF for the quoted-printable
encoding:

    qp-line := *(qp-segment transport-padding CRLF)
               qp-part transport-padding
    qp-segment := qp-section *(SPACE / TAB) "="
    transport-padding := *LWSP-char
                          ; Composers MUST NOT generate
                          ; non-zero length transport
                          ; padding, but receivers MUST
                          ; be able to handle padding
                          ; added by message transports.

RFC 822 defines LWSP-char as:

    LWSP-char   =  SPACE / HTAB

Dovecot's imaptest contains such a message in
src/tests/fetch-binary-mime-qp.mbox.

Fixes #70952

src/mime/quotedprintable/reader.go

@@ -66,6 +66,7 @@ var (
 	crlf       = []byte("\r\n")
 	lf         = []byte("\n")
 	softSuffix = []byte("=")
+	lwspChar   = " \t"
 )
 
 // Read reads and decodes quoted-printable data from the underlying reader.
@@ -92,7 +93,7 @@ func (r *Reader) Read(p []byte) (n int, err error) {
 			wholeLine := r.line
 			r.line = bytes.TrimRightFunc(wholeLine, isQPDiscardWhitespace)
 			if bytes.HasSuffix(r.line, softSuffix) {
-				rightStripped := wholeLine[len(r.line):]
+				rightStripped := bytes.TrimLeft(wholeLine[len(r.line):], lwspChar)
 				r.line = r.line[:len(r.line)-1]
 				if !bytes.HasPrefix(rightStripped, lf) && !bytes.HasPrefix(rightStripped, crlf) &&
 					!(len(rightStripped) == 0 && len(r.line) > 0 && r.rerr == io.EOF) {

src/mime/quotedprintable/reader_test.go

@@ -66,6 +66,10 @@ func TestReader(t *testing.T) {
 			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: ’"},
+
+		// Transport padding
+		{in: "foo= \r\nbar", want: "foobar"},
+		{in: "foo=\t \r\nbar", want: "foobar"},
 	}
 	for _, tt := range tests {
 		var buf strings.Builder
@@ -199,13 +203,13 @@ func TestExhaustive(t *testing.T) {
 	}
 	slices.Sort(outcomes)
 	got := strings.Join(outcomes, "\n")
-	want := `OK: 28934
-invalid bytes after =: 3949
-quotedprintable: invalid hex byte 0x0d: 2048
+	want := `OK: 30638
+invalid bytes after =: 2243
+quotedprintable: invalid hex byte 0x0d: 2050
 unexpected EOF: 194`
 	if testing.Short() {
-		want = `OK: 896
-invalid bytes after =: 100
+		want = `OK: 935
+invalid bytes after =: 61
 quotedprintable: invalid hex byte 0x0d: 26
 unexpected EOF: 3`
 	}