Add default body background color

Webpages without a background color on the body are saved by phantomjs with a transparent background.
But when images are cropped by ImageMagick convert, the transparency is lost and is replaced by a white background. And because only one of the images is cropped, you get a diff with all the transparent background in blue.

I see 3 possibilities :
1/ Save images with a white background if no background color is specified
2/ Adapt the ImageMagick convert call to preserve transparency
3/ Call convert on the 2 images, even if the base image is already sized correctly

I think 1 is just fine, as white is usually the default background color.
2 might be better but I couldn't find how to do it.

This is a pull request for 1.