Fixed getting the location header

The old response._headers.get() doesn't work anymore with newer Django.
This patch fixes it.

Change-Id: I8c87804feef9909cd4697f01ceea0f5504347727

heat_dashboard/test/helpers.py

@@ -200,7 +200,10 @@ class TestCase(horizon_helpers.TestCase):
         processing the view which is redirected to.
         """
         if django.VERSION >= (1, 9):
-            loc = str(response._headers.get('location', None)[1])
+            if response.has_header('location'):
+                loc = response['location']
+            else:
+                loc = ''
             loc = http.urlunquote(loc)
             expected_url = http.urlunquote(expected_url)
             self.assertEqual(loc, expected_url)