Fixed a bug in AddAlternativeWriter

AddAlternativeWriter was replacing the message body instead
of adding a body part.

Fixes #44

message.go

@@ -197,26 +197,22 @@ func (m *Message) SetBody(contentType, body string) {
 //
 // More info: http://en.wikipedia.org/wiki/MIME#Alternative
 func (m *Message) AddAlternative(contentType, body string) {
-	m.parts = append(m.parts,
-		part{
-			header: m.getPartHeader(contentType),
-			copier: func(w io.Writer) error {
-				_, err := io.WriteString(w, body)
-				return err
-			},
+	m.parts = append(m.parts, part{
+		header: m.getPartHeader(contentType),
+		copier: func(w io.Writer) error {
+			_, err := io.WriteString(w, body)
+			return err
 		},
-	)
+	})
 }
 
 // AddAlternativeWriter adds an alternative part to the message. It can be
 // useful with the text/template or html/template packages.
 func (m *Message) AddAlternativeWriter(contentType string, f func(io.Writer) error) {
-	m.parts = []part{
-		{
-			header: m.getPartHeader(contentType),
-			copier: f,
-		},
-	}
+	m.parts = append(m.parts, part{
+		header: m.getPartHeader(contentType),
+		copier: f,
+	})
 }
 
 func (m *Message) getPartHeader(contentType string) header {

message_test.go

@@ -63,29 +63,6 @@ func TestMessage(t *testing.T) {
 	testMessage(t, m, 0, want)
 }
 
-func TestBodyWriter(t *testing.T) {
-	m := NewMessage()
-	m.SetHeader("From", "from@example.com")
-	m.SetHeader("To", "to@example.com")
-	m.AddAlternativeWriter("text/plain", func(w io.Writer) error {
-		_, err := w.Write([]byte("Test message"))
-		return err
-	})
-
-	want := &message{
-		from: "from@example.com",
-		to:   []string{"to@example.com"},
-		content: "From: from@example.com\r\n" +
-			"To: to@example.com\r\n" +
-			"Content-Type: text/plain; charset=UTF-8\r\n" +
-			"Content-Transfer-Encoding: quoted-printable\r\n" +
-			"\r\n" +
-			"Test message",
-	}
-
-	testMessage(t, m, 0, want)
-}
-
 func TestCustomMessage(t *testing.T) {
 	m := NewMessage(SetCharset("ISO-8859-1"), SetEncoding(Base64))
 	m.SetHeaders(map[string][]string{
@@ -191,6 +168,42 @@ func TestAlternative(t *testing.T) {
 	testMessage(t, m, 1, want)
 }
 
+func TestBodyWriter(t *testing.T) {
+	m := NewMessage()
+	m.SetHeader("From", "from@example.com")
+	m.SetHeader("To", "to@example.com")
+	m.AddAlternativeWriter("text/plain", func(w io.Writer) error {
+		_, err := w.Write([]byte("Test message"))
+		return err
+	})
+	m.AddAlternativeWriter("text/html", func(w io.Writer) error {
+		_, err := w.Write([]byte("Test HTML"))
+		return err
+	})
+
+	want := &message{
+		from: "from@example.com",
+		to:   []string{"to@example.com"},
+		content: "From: from@example.com\r\n" +
+			"To: to@example.com\r\n" +
+			"Content-Type: multipart/alternative; boundary=_BOUNDARY_1_\r\n" +
+			"\r\n" +
+			"--_BOUNDARY_1_\r\n" +
+			"Content-Type: text/plain; charset=UTF-8\r\n" +
+			"Content-Transfer-Encoding: quoted-printable\r\n" +
+			"\r\n" +
+			"Test message\r\n" +
+			"--_BOUNDARY_1_\r\n" +
+			"Content-Type: text/html; charset=UTF-8\r\n" +
+			"Content-Transfer-Encoding: quoted-printable\r\n" +
+			"\r\n" +
+			"Test HTML\r\n" +
+			"--_BOUNDARY_1_--\r\n",
+	}
+
+	testMessage(t, m, 1, want)
+}
+
 func TestAttachmentOnly(t *testing.T) {
 	m := NewMessage()
 	m.SetHeader("From", "from@example.com")