Replace CRLF with LF in Writer

This is what the mbox format uses, despite the fact that the Internet Message
Format uses CRLF.

Closes: #20

writer.go

@@ -39,7 +39,14 @@ func (mw *messageWriter) Write(p []byte) (int, error) {
 		var l []byte
 		l, b = b[:i+1], b[i+1:]
 
-		n, err := mw.writeLine(l)
+		n := len(l)
+		// Replace CRLF with LF
+		if len(l) > 1 && l[len(l)-2] == '\r' {
+			l = l[:len(l)-2]
+			l = append(l, '\n')
+		}
+
+		_, err := mw.writeLine(l)
 		N += n
 		if err != nil {
 			return N, err
@@ -54,7 +61,7 @@ func (mw *messageWriter) Close() error {
 		return err
 	}
 
-	_, err := mw.w.Write([]byte("\r\n\r\n"))
+	_, err := mw.w.Write([]byte("\n\n"))
 	return err
 }
 
@@ -94,7 +101,7 @@ func (w *Writer) CreateMessage(from string, t time.Time) (io.Writer, error) {
 	}
 	date := t.Format(time.ANSIC)
 
-	line := "From " + from + " " + date + "\r\n"
+	line := "From " + from + " " + date + "\n"
 	if _, err := io.WriteString(w.w, line); err != nil {
 		return nil, err
 	}

writer_test.go

@@ -55,17 +55,17 @@ Bye.`,
 		},
 		{
 			"Thu, 02 Jan 2015 00:00:01 +0100",
-			`Date: Thu, 02 Jan 2015 00:00:01 +0100
-
-This is another simple test.
-
-Another line.
-
+			`Date: Thu, 02 Jan 2015 00:00:01 +0100` + "\r" + `
+` + "\r" + `
+This is another simple test.` + "\r" + `
+` + "\r" + `
+Another line.` + "\r" + `
+` + "\r" + `
 Bye.`,
 		},
 	}
 
-	expected := `From ???@??? Thu Jan  1 00:00:01 2015` + "\r" + `
+	expected := `From ???@??? Thu Jan  1 00:00:01 2015
 Date: Thu, 01 Jan 2015 00:00:01 +0100
 
 This is a simple test.
@@ -74,17 +74,17 @@ And, by the way, this is how a "From" line is escaped in mboxo format:
 
 >From Herp Derp with love.
 
-Bye.` + "\r" + `
-` + "\r" + `
-From ???@??? Fri Jan  2 00:00:01 2015` + "\r" + `
+Bye.
+
+From ???@??? Fri Jan  2 00:00:01 2015
 Date: Thu, 02 Jan 2015 00:00:01 +0100
 
 This is another simple test.
 
 Another line.
 
-Bye.` + "\r" + `
-` + "\r" + `
+Bye.
+
 `
 
 	s := testWriter(t, messages)