Avoid meaningless comparison that leads to a TypeError

The "requests" module tries to do this comparison in raise_for_status():

    400 <= self.status_code < 500

with "self.status_code" being "None". In Python2, it's a valid comparison, but
in Python3, it triggers a TypeError. We fix it by giving a valid default value
to "self.status_code".

Change-Id: I9ea9ecfb698546a7f6526464aa528441bb7c1ccd
Change assertEquals to assertIsNone

Some tests were using assertEquals where they should have used
assertIsNone. assertEquals doesn't distinguish False from None.

Change-Id: Id7dc1075fbaaa9a6dbff5b05ab587df39c79c180
Merge "Avoid meaningless comparison that leads to a TypeError"