fix colorspace's problem

[MrNocole]

when convert a jpg(or others, colorspace is GRAY). will raise an exception.
example:
use these python code to convert a normal pic to gray colorspace.
`from PIL import Image

image = Image.open("image.jpg")

gray_image = image.convert("L")

gray_image.save("gray_image.jpg")`
then use HEIF
./HEIF gray_image.jpg

so we need use default colorspace ( I think rgb ) is suitable when colorspace is not supported.