Fix set_background_color_rgb for Imagemagick 7
Fix set_background_color_rgb for Imagemagick 7

The line `clone.image.alpha_channel = False` (a workaround for alpha_channel failing to update on Imagemagick <= 6) breaks on Imagemagick 7 - presumably because False is not a legal value to set - so it becomes True when read back.

gasman/Willow,gasman/Willow