GODT-1044: fix header lines parsing

pkg/message/header.go

@@ -35,11 +35,15 @@ func HeaderLines(header []byte) [][]byte {
 	)
 
 	forEachLine(bufio.NewReader(bytes.NewReader(header)), func(line []byte) {
+		l := bytes.SplitN(line, []byte(`: `), 2)
+		isLineContinuation := quote%2 != 0 || // no quotes opened
+			len(l) != 2 || // it doesn't have colon
+			(len(l) == 2 && !bytes.Equal(bytes.TrimSpace(l[0]), l[0])) // has white space in front of header field
 		switch {
 		case len(bytes.TrimSpace(line)) == 0:
 			lines = append(lines, line)
 
-		case quote%2 != 0, len(bytes.SplitN(line, []byte(`: `), 2)) != 2:
+		case isLineContinuation:
 			if len(lines) > 0 {
 				lines[len(lines)-1] = append(lines[len(lines)-1], line...)
 			} else {

pkg/message/header_test.go

@@ -24,14 +24,19 @@ import (
 )
 
 func TestHeaderLines(t *testing.T) {
-	const header = "To: somebody\r\nFrom: somebody else\r\nSubject: this is\r\n\ta multiline field\r\n\r\n"
-
-	assert.Equal(t, [][]byte{
+	want := [][]byte{
 		[]byte("To: somebody\r\n"),
 		[]byte("From: somebody else\r\n"),
-		[]byte("Subject: this is\r\n\ta multiline field\r\n"),
+		[]byte("Subject: RE: this is\r\n\ta multiline field: with colon\r\n\tor: many: more: colons\r\n"),
+		[]byte("X-Special: \r\n\tNothing on the first line\r\n\tbut has something on the other lines\r\n"),
 		[]byte("\r\n"),
-	}, HeaderLines([]byte(header)))
+	}
+	var header []byte
+	for _, line := range want {
+		header = append(header, line...)
+	}
+
+	assert.Equal(t, want, HeaderLines(header))
 }
 
 func TestHeaderLinesMultilineFilename(t *testing.T) {