Fixes #8

We were using `io.Copy` to write to the body string/alternative string to the io.Writer. This placed the byte position of the buffer to be at the EOF after the first `WriteTo()` call leaving the output of a 2nd call to `WriteTo()` empty.

msg.go

@@ -390,8 +390,8 @@ func (m *Msg) GetRecipients() ([]string, error) {
 func (m *Msg) SetBodyString(ct ContentType, b string, o ...PartOption) {
 	buf := bytes.NewBufferString(b)
 	w := func(w io.Writer) (int64, error) {
-		nb, err := io.Copy(w, buf)
-		return nb, err
+		nb, err := w.Write(buf.Bytes())
+		return int64(nb), err
 	}
 	m.SetBodyWriter(ct, w, o...)
 }
@@ -407,8 +407,8 @@ func (m *Msg) SetBodyWriter(ct ContentType, w func(io.Writer) (int64, error), o
 func (m *Msg) AddAlternativeString(ct ContentType, b string, o ...PartOption) {
 	buf := bytes.NewBufferString(b)
 	w := func(w io.Writer) (int64, error) {
-		nb, err := io.Copy(w, buf)
-		return nb, err
+		nb, err := w.Write(buf.Bytes())
+		return int64(nb), err
 	}
 	m.AddAlternativeWriter(ct, w, o...)
 }

msg_test.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"net/mail"
+	"strings"
 	"testing"
 	"time"
 )
@@ -1244,3 +1245,30 @@ func TestMsg_appendFile(t *testing.T) {
 		t.Errorf("appendFile() failed. Expected length: %d, got: %d", 2, len(fl))
 	}
 }
+
+// TestMsg_multipleWrites tests multiple executions of WriteTo on the Msg
+func TestMsg_multipleWrites(t *testing.T) {
+	ts := "XXX_UNIQUE_STRING_XXX"
+	wbuf := bytes.Buffer{}
+	m := NewMsg()
+	m.SetBodyString(TypeTextPlain, ts)
+
+	// First WriteTo()
+	_, err := m.WriteTo(&wbuf)
+	if err != nil {
+		t.Errorf("failed to write body to buffer: %s", err)
+	}
+	if !strings.Contains(wbuf.String(), ts) {
+		t.Errorf("first WriteTo() body does not contain unique string: %s", ts)
+	}
+
+	// Second WriteTo()
+	wbuf.Reset()
+	_, err = m.WriteTo(&wbuf)
+	if err != nil {
+		t.Errorf("failed to write body to buffer: %s", err)
+	}
+	if !strings.Contains(wbuf.String(), ts) {
+		t.Errorf("second WriteTo() body does not contain unique string: %s", ts)
+	}
+}