Fixed a panic when a Message contains a nil header

Fixes #41
Fixes #42

message_test.go

@@ -477,6 +477,22 @@ func TestBase64LineLength(t *testing.T) {
 	testMessage(t, m, 0, want)
 }
 
+func TestEmptyHeader(t *testing.T) {
+	m := NewMessage()
+	m.SetHeaders(map[string][]string{
+		"From":    {"from@example.com"},
+		"X-Empty": nil,
+	})
+
+	want := &message{
+		from: "from@example.com",
+		content: "From: from@example.com\r\n" +
+			"X-Empty: \r\n",
+	}
+
+	testMessage(t, m, 0, want)
+}
+
 func testMessage(t *testing.T, m *Message, bCount int, want *message) {
 	err := Send(stubSendMail(t, bCount, want), m)
 	if err != nil {

writeto.go

@@ -154,23 +154,16 @@ func (w *messageWriter) writeString(s string) {
 	w.n += int64(n)
 }
 
-func (w *messageWriter) writeStrings(a []string, sep string) {
-	if len(a) > 0 {
-		w.writeString(a[0])
-		if len(a) == 1 {
-			return
-		}
-	}
-	for _, s := range a[1:] {
-		w.writeString(sep)
-		w.writeString(s)
-	}
-}
-
 func (w *messageWriter) writeHeader(k string, v ...string) {
 	w.writeString(k)
 	w.writeString(": ")
-	w.writeStrings(v, ", ")
+	if len(v) > 0 {
+		w.writeString(v[0])
+		for _, s := range v[1:] {
+			w.writeString(", ")
+			w.writeString(s)
+		}
+	}
 	w.writeString("\r\n")
 }