CreateMessage writer can return too many bytes written

[magical]

The writer returned by CreateMessage can sometimes claim to have written more bytes than it was given. In particular, if a Write is done while there is an incomplete line buffered, the returned n will include the length of the buffered bytes as well as the provided bytes.

Here's a small program demonstrating the problem:

package main

import (
	"fmt"
	"io"
	"io/ioutil"
	"time"

	mbox "github.com/emersion/go-mbox"
)

func main() {
	mboxWriter := mbox.NewWriter(ioutil.Discard)
	w, err := mboxWriter.CreateMessage("-", time.Time{})
	if err != nil {
		panic(err)
	}
	write(w, []byte("some text   "))
	write(w, []byte("end of line\n"))
}

func write(w io.Writer, b []byte) {
	n, err := w.Write(b)
	fmt.Printf("Write(%d bytes) wrote %d bytes\n", len(b), n)
	if err != nil {
		fmt.Println("error:", err)
	}
}

Playground: https://play.golang.org/p/uwFl_72yx0p

This prints

Write(12 bytes) wrote 12 bytes
Write(12 bytes) wrote 24 bytes

but it should print

Write(12 bytes) wrote 12 bytes
Write(12 bytes) wrote 12 bytes