Update models.py

When using the request middleware, if the user is anonymous an error is raised stating that the 'history_user' attribute must be an instance of User. As this field can be null, get_history_user has been updated to return None if the user is not authenticated. Also made the returning of None explicit.

Author: RossLote

simple_history/models.py

@@ -233,9 +233,11 @@ def get_history_user(self, instance):
             return instance._history_user
         except AttributeError:
             try:
-                return self.thread.request.user
+                if self.thread.request.user.is_authenticated():
+                    return self.thread.request.user
+                return None
             except AttributeError:
-                return
+                return None
 
 
 class ForeignKeyMixin(object):