Python 3: the content of a FakeResponse must be bytes

Encode '_content' if necessary.

Change-Id: I25c1e1cd5330f0519bf062be840045d0ef520b28

openstackclient/tests/common/test_restapi.py

@@ -19,6 +19,7 @@ import json
 import mock
 
 import requests
+import six
 
 from openstackclient.common import restapi
 from openstackclient.tests import utils
@@ -62,6 +63,8 @@ class FakeResponse(requests.Response):
 
         self.headers.update(headers)
         self._content = json.dumps(data)
+        if not isinstance(self._content, six.binary_type):
+            self._content = self._content.encode()
 
 
 @mock.patch('openstackclient.common.restapi.requests.Session')