Fix error encoding datetime objects in the event track logger backend

The function json.dumps was being called with incorrect arguments,
causing an infinite recursion when serializing an event containing
datetime objects

common/djangoapps/track/backends/logger.py

@@ -33,7 +33,7 @@ class LoggerBackend(BaseBackend):
         self.event_logger = logging.getLogger(name)
 
     def send(self, event):
-        event_str = json.dumps(event, default=DateTimeJSONEncoder)
+        event_str = json.dumps(event, cls=DateTimeJSONEncoder)
 
         # TODO: remove trucation of the serialized event, either at a
         # higher level during the emittion of the event, or by

common/djangoapps/track/backends/tests/test_logger.py

@@ -2,6 +2,7 @@ from __future__ import absolute_import
 
 import json
 import logging
+import datetime
 
 from django.test import TestCase
 
@@ -26,16 +27,25 @@ class TestLoggerBackend(TestCase):
         # Send a couple of events and check if they were recorded
         # by the logger. The events are serialized to JSON.
 
-        event = {'test': True}
-        event_as_json = json.dumps(event)
+        event = {
+            'test': True,
+            'time': datetime.datetime(2012, 05, 01, 07, 27, 01, 200),
+            'date': datetime.date(2012, 05, 07),
+        }
 
         self.backend.send(event)
         self.backend.send(event)
 
-        self.assertEqual(
-            self.handler.messages['info'],
-            [event_as_json, event_as_json]
-        )
+        saved_events = [json.loads(e) for e in self.handler.messages['info']]
+
+        unpacked_event = {
+            'test': True,
+            'time': '2012-05-01T07:27:01.000200+00:00',
+            'date': '2012-05-07'
+        }
+
+        self.assertEqual(saved_events[0], unpacked_event)
+        self.assertEqual(saved_events[1], unpacked_event)
 
 
 class MockLoggingHandler(logging.Handler):