FakeResponse: use a default status code

When running some tests from test_restapi.py, the following error happens:

    TypeError: unorderable types: NoneType() < int()

In Python 2, comparing NoneType and integers is possible:

    >>> None < 2
    True

But in Python 3, it's not allowed. Fix this by using a default status code.

Change-Id: Ic0fad5c68f3bf2dd8a2b98423549903f982192c9

Author: CyrilRoelandteNovance

openstackclient/tests/common/test_restapi.py

@@ -55,7 +55,7 @@
 
 
 class FakeResponse(requests.Response):
-    def __init__(self, headers={}, status_code=None, data=None, encoding=None):
+    def __init__(self, headers={}, status_code=200, data=None, encoding=None):
         super(FakeResponse, self).__init__()
 
         self.status_code = status_code