Created Unassigned: Coalesce adds border [1034]

First of all thank you for this great library!

I am using the latest version of Magick.NET to open, coalesce and save the following animated gif image:

http://img3.fotos-hochladen.net/uploads/66901gqceyar.gif (also attached to this issue).

```
using (var originalImage = new MagickImageCollection(originalImageStream))
{
originalImage.Coalesce();
originalImage.Write(targetStream);
}
```

The coalesced image now has a black border at the right and the bottom.