incorrect video after VideoWriter if is_color == false (Bug #1615)

Sample:

 1    VideoCapture cap(argv[1]);
 2
 3    if (!cap.isOpened())
 4      return 1;
 5
 6    Ptr<VideoWriter> vw = 0;
 7
 8    while(true)
 9    {
10        Mat frame;
11        cap>>frame;
12        if (frame.empty())
13            break;
14
15        if (!vw)
16        {
17           vw = new VideoWriter(argv[2], CV_FOURCC('P','I','M','1'), 25, frame.size(), false);
18           if (!vw->isOpened())
19               break;
20        }
21
22        Mat gray;
23        cvtColor(frame, gray, CV_BGR2GRAY);
24
25        *vw << gray;
26
27        imshow("gray", gray);
28        waitKey(1);
29    }
30    vw->~VideoWriter();