Set default value to None for --image_max_width during prediction

During prediction, images are resized with:

if image_max_width:
    _, w, _ = read_image(image, scale=1).shape
    ratio = image_max_width / w
    im = read_image(image, ratio)
else:
    im = read_image(image, scale=scale)

But since the current default value of --image_max_width is set to 1800 pixels, the scale parameter is useless. In my case, I used --scale 1, but my images ended up being resized to 1800 pixels anyway.

We need to set the default --image_max_width value to None.