Changed the internal representation of a part

message.go

@@ -26,7 +26,7 @@ type Message struct {
 type header map[string][]string
 
 type part struct {
-	contentType string
+	header header
 	copier func(io.Writer) error
 }
 
@@ -201,7 +201,7 @@ func (m *Message) DelHeader(field string) {
 func (m *Message) SetBody(contentType, body string) {
 	m.parts = []part{
 		{
-			contentType: contentType,
+			header: m.getPartHeader(contentType),
 			copier: func(w io.Writer) error {
 				_, err := io.WriteString(w, body)
 				return err
@@ -223,7 +223,7 @@ func (m *Message) SetBody(contentType, body string) {
 func (m *Message) AddAlternative(contentType, body string) {
 	m.parts = append(m.parts,
 		part{
-			contentType: contentType,
+			header: m.getPartHeader(contentType),
 			copier: func(w io.Writer) error {
 				_, err := io.WriteString(w, body)
 				return err
@@ -244,12 +244,19 @@ func (m *Message) AddAlternative(contentType, body string) {
 func (m *Message) AddAlternativeWriter(contentType string, f func(io.Writer) error) {
 	m.parts = []part{
 		{
-			contentType: contentType,
+			header: m.getPartHeader(contentType),
 			copier: f,
 		},
 	}
 }
 
+func (m *Message) getPartHeader(contentType string) header {
+	return map[string][]string{
+		"Content-Type":              {contentType + "; charset=" + m.charset},
+		"Content-Transfer-Encoding": {string(m.encoding)},
+	}
+}
+
 // A File represents a file that can be attached or embedded in an email.
 type File struct {
 	// Name represents the base name of the file. If the file is attached to the

writeto.go

@@ -38,11 +38,7 @@ func (w *messageWriter) writeMessage(m *Message) {
 		w.openMultipart("alternative")
 	}
 	for _, part := range m.parts {
-		contentType := part.contentType + "; charset=" + m.charset
-		w.writeHeaders(map[string][]string{
-			"Content-Type":              {contentType},
-			"Content-Transfer-Encoding": {string(m.encoding)},
-		})
+		w.writeHeaders(part.header)
 		w.writeBody(part.copier, m.encoding)
 	}
 	if m.hasAlternativePart() {