Test fialures

[WYSIATI]

When I run python can_test.py in canard/test with python 2 and 3, I always have failures because of something like this [1, 2, 3, 4, 5, 6, 0, 0] != [1, 2, 3, 4, 5, 6, None, None].

It looks like when you create a 8-byte frame with only 6 given integers, the rest would be 0 instead of None, if I am right, I will create a pull request to correct this.