Py3k compliance: check for bytes when making a string

When faking stdout in tests we must account for the case when
"strings" might be bytes, which python3 cares about.

Change-Id: I0e9349c86371055eed479799d39ab6adbcad6f95
Closes-Bug: #1499004

neutronclient/tests/unit/test_cli20.py

@@ -73,6 +73,12 @@ class FakeStdout(object):
     def make_string(self):
         result = ''
         for line in self.content:
+            if six.PY3:
+                if isinstance(line, bytes):
+                    try:
+                        line = line.decode(encoding='utf-8')
+                    except UnicodeError:
+                        pass
             result = result + line
         return result