What steps will reproduce the problem?
1. Download the example app, samples/translate/main.py.
2. Edit the request such that the optional argument source=None
  service.translations().list(source=None, target='fr', q=['car']).execute()

What is the expected output? What do you see instead?
Printing the request URI reveals that None is encoded as the string "None" as 
opposed to excluding the relevant argument.

What version of the product are you using? On what operating system?
2.0beta8

Please provide any additional information below.