write base64 message parts with 76 data bytes on a line instead of 78

As required by RFC 2045 (MIME). The 78 byte lines work in practice, except that
SpamAssassin has rules that give messages with 78-byte lines spam points.

Mentioned by kjetilho on irc.

moxio/base64writer_test.go

@@ -13,7 +13,7 @@ func TestBase64Writer(t *testing.T) {
 	err = bw.Close()
 	tcheckf(t, err, "close")
 	s := sb.String()
-	exp := "MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIzNDU2Nz\r\ng5MDEyMzQ1Njc4OQ==\r\n"
+	exp := "MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIzNDU2\r\nNzg5MDEyMzQ1Njc4OQ==\r\n"
 	if s != exp {
 		t.Fatalf("base64writer, got %q, expected %q", s, exp)
 	}