thumbnail resize bug?

I found some interesting behaviour in the Imagick thumbnail creation function. Namely that when I resized square sized images into smaller sized square images (like 300x300 into 220x220) they lost a pixel (like they became 219x220). I think it might be due the fact that in `imagick_helpers.c` one side of the image is _always_ calculated from the original size and a ratio which is a `double`. So I added another branch to the `if` for square images where the size isn't calculated, the destined width and height is used.

**CAUTION**
I couldn't test my code. Use it with care!