to_cv_image does not work with mode=None

[Domibor]

Problem:
to_cv_image throws an error if mode=None, but it is marked as an optional argument.

What should happen:
If mode=None the image should not be converted to any other color space (should return npimg directly)

[hityzy1122]

Problem:
to_cv_image throws an error if mode=None, but it is marked as an optional argument.

What should happen:
If mode=None the image should not be converted to any other color space (should return npimg directly)

I have fixed some bugs as your suggestion. Thanks a lot!