TestResponse: use a default status code

The status_code field is compared to integers in the requests library, so it
has to be set to an integer, not to None. This works in Python 2 because back
then, one could compare an int to NoneType.

Partial-Bug: 1287323
Change-Id: I7138ceca36692fdf5948c4d6f74186f89c2709f0
Merge "TestResponse: use a default status code"