Fix unicode error when assertMessageCount() fails

When assertMessageCount() fails, we display the list of messages that
were included in the response. However because the messages are lazily
translated by default, this would fail and show the following error
instead:

 TypeError: sequence item 0: expected string, __proxy__ found

Change-Id: I1d2ed97b66428e0ee1aac783c8370ab3ea665b29
Closes-Bug: #1315031

horizon/test/helpers.py

@@ -31,6 +31,7 @@ from django.core.handlers import wsgi
 from django import http
 from django import test as django_test
 from django.test.client import RequestFactory  # noqa
+from django.utils.encoding import force_unicode
 from django.utils import unittest
 
 LOG = logging.getLogger(__name__)
@@ -154,7 +155,8 @@ class TestCase(django_test.TestCase):
 
         # Otherwise, make sure we got the expected messages.
         for msg_type, count in kwargs.items():
-            msgs = [m.message for m in messages if msg_type in m.tags]
+            msgs = [force_unicode(m.message)
+                    for m in messages if msg_type in m.tags]
             assert len(msgs) == count, \
                    "%s messages not as expected: %s" % (msg_type.title(),
                                                         ", ".join(msgs))