use django facilities to get hostname from request (#16525)

This is required as SERVER_NAME may not be used in some uwsgi
configuration, and HTTP_HOST should be used instead.

| Nginx maps the $server_name variable to the first server_name you define.
| In your case you have two solutions: read HTTP_HOST instead of SERVER_NAME
| in your app or set SERVER_NAME to $http_host in uwsgi_params
  -- http://lists.unbit.it/pipermail/uwsgi/2010-August/000571.html

The HttpRequest.get_host method handles those cases and more.

mellon/middleware.py

@@ -35,7 +35,7 @@ class PassiveAuthenticationMiddleware(object):
             # get the common domain or guess
             common_domain = app_settings.OPENED_SESSION_COOKIE_DOMAIN
             if not common_domain:
-                common_domain = request.META['SERVER_NAME'].split('.', 1)[1]
+                common_domain = request.get_host().split('.', 1)[1]
                 assert '.' in common_domain  # if domain is xxx.com explode !
             params = {
                 'next': request.build_absolute_uri(),