Sanitize create_container input

Client.create_container takes a volumes_from argument that should be: a
string, a list of strings, or None. If an empty list or dict is passed
instead, the docker daemon returns a 406 Client Error: Not Acceptable
Add test to check for the absence of volumes_from
Missed empty line for flake8
Merge pull request #198 from fcoelho/fix-volumes-from

Sanitize create_container input for volumes_from