Provided a way to specify the encoding of a message part

This commit introduces a minor backward-compatibility break:
Message.SetBody, Message.AddAlternative and
Message.AddAlternativeWriter now accept a new variadic argument.

Since this argument is optional, in almost all cases there is
nothing to change. That is why Gomail version number wasn't bumped.

Fixes #47

example_test.go

@@ -213,3 +213,7 @@ func ExampleSetCharset() {
 func ExampleSetEncoding() {
 	m = gomail.NewMessage(gomail.SetEncoding(gomail.Base64))
 }
+
+func ExampleWithEncoding() {
+	m.SetBody("text/plain", "Hello!", gomail.SetPartEncoding(gomail.Unencoded))
+}

message.go

@@ -11,7 +11,7 @@ import (
 // Message represents an email.
 type Message struct {
 	header      header
-	parts       []part
+	parts       []*part
 	attachments []*file
 	embedded    []*file
 	charset     string
@@ -23,8 +23,9 @@ type Message struct {
 type header map[string][]string
 
 type part struct {
-	header header
-	copier func(io.Writer) error
+	contentType string
+	copier      func(io.Writer) error
+	encoding    Encoding
 }
 
 // NewMessage creates a new message. It uses UTF-8 and quoted-printable encoding
@@ -179,16 +180,8 @@ func (m *Message) GetHeader(field string) []string {
 
 // SetBody sets the body of the message. It replaces any content previously set
 // by SetBody, AddAlternative or AddAlternativeWriter.
-func (m *Message) SetBody(contentType, body string) {
-	m.parts = []part{
-		{
-			header: m.getPartHeader(contentType),
-			copier: func(w io.Writer) error {
-				_, err := io.WriteString(w, body)
-				return err
-			},
-		},
-	}
+func (m *Message) SetBody(contentType, body string, settings ...PartSetting) {
+	m.parts = []*part{m.newPart(contentType, newCopier(body), settings)}
 }
 
 // AddAlternative adds an alternative part to the message.
@@ -197,30 +190,48 @@ func (m *Message) SetBody(contentType, body string) {
 // version for backward compatibility. AddAlternative appends the new part to
 // the end of the message. So the plain text part should be added before the
 // HTML part. See 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
-		},
-	})
+func (m *Message) AddAlternative(contentType, body string, settings ...PartSetting) {
+	m.AddAlternativeWriter(contentType, newCopier(body), settings...)
+}
+
+func newCopier(s string) func(io.Writer) error {
+	return func(w io.Writer) error {
+		_, err := io.WriteString(w, s)
+		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 = append(m.parts, part{
-		header: m.getPartHeader(contentType),
-		copier: f,
-	})
+func (m *Message) AddAlternativeWriter(contentType string, f func(io.Writer) error, settings ...PartSetting) {
+	m.parts = append(m.parts, m.newPart(contentType, f, settings))
 }
 
-func (m *Message) getPartHeader(contentType string) header {
-	return map[string][]string{
-		"Content-Type":              {contentType + "; charset=" + m.charset},
-		"Content-Transfer-Encoding": {string(m.encoding)},
+func (m *Message) newPart(contentType string, f func(io.Writer) error, settings []PartSetting) *part {
+	p := &part{
+		contentType: contentType,
+		copier:      f,
+		encoding:    m.encoding,
 	}
+
+	for _, s := range settings {
+		s(p)
+	}
+
+	return p
+}
+
+// A PartSetting can be used as an argument in Message.SetBody,
+// Message.AddAlternative or Message.AddAlternativeWriter to configure the part
+// added to a message.
+type PartSetting func(*part)
+
+// SetPartEncoding sets the encoding of the part added to the message. By
+// default, parts use the same encoding than the message.
+func SetPartEncoding(e Encoding) PartSetting {
+	return PartSetting(func(p *part) {
+		p.encoding = e
+	})
 }
 
 type file struct {

message_test.go

@@ -168,6 +168,36 @@ func TestAlternative(t *testing.T) {
 	testMessage(t, m, 1, want)
 }
 
+func TestPartSetting(t *testing.T) {
+	m := NewMessage()
+	m.SetHeader("From", "from@example.com")
+	m.SetHeader("To", "to@example.com")
+	m.SetBody("text/plain; format=flowed", "¡Hola, señor!", SetPartEncoding(Unencoded))
+	m.AddAlternative("text/html", "¡<b>Hola</b>, <i>señor</i>!</h1>")
+
+	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; format=flowed; charset=UTF-8\r\n" +
+			"Content-Transfer-Encoding: 8bit\r\n" +
+			"\r\n" +
+			"¡Hola, señor!\r\n" +
+			"--_BOUNDARY_1_\r\n" +
+			"Content-Type: text/html; charset=UTF-8\r\n" +
+			"Content-Transfer-Encoding: quoted-printable\r\n" +
+			"\r\n" +
+			"=C2=A1<b>Hola</b>, <i>se=C3=B1or</i>!</h1>\r\n" +
+			"--_BOUNDARY_1_--\r\n",
+	}
+
+	testMessage(t, m, 1, want)
+}
+
 func TestBodyWriter(t *testing.T) {
 	m := NewMessage()
 	m.SetHeader("From", "from@example.com")

writeto.go

@@ -38,8 +38,7 @@ func (w *messageWriter) writeMessage(m *Message) {
 		w.openMultipart("alternative")
 	}
 	for _, part := range m.parts {
-		w.writeHeaders(part.header)
-		w.writeBody(part.copier, m.encoding)
+		w.writePart(part, m.charset)
 	}
 	if m.hasAlternativePart() {
 		w.closeMultipart()
@@ -104,6 +103,14 @@ func (w *messageWriter) closeMultipart() {
 	}
 }
 
+func (w *messageWriter) writePart(p *part, charset string) {
+	w.writeHeaders(map[string][]string{
+		"Content-Type":              {p.contentType + "; charset=" + charset},
+		"Content-Transfer-Encoding": {string(p.encoding)},
+	})
+	w.writeBody(p.copier, p.encoding)
+}
+
 func (w *messageWriter) addFiles(files []*file, isAttachment bool) {
 	for _, f := range files {
 		if _, ok := f.Header["Content-Type"]; !ok {