title: __init__() got an unexpected keyword argument 'default' in json/__init__.py in dumps
body: <p>I have a django server running, and, out of nowhere, I got this error. I really have no idea what changes led to this.</p>
  
  <p>I'm trying to solve this for hours, but I can't find why this is happening.</p>
  
  <p>Please give me a few ideas.</p>
  
  <p>Ask me if you want a specific part of my code. Although the Python Path is not mentioning any of my files..</p>
  
  <p><img src="https://i.stack.imgur.com/v2KWU.png" alt="Bug python"></p>
  
  <p><strong>Full traceback :</strong></p>
  
  <pre><code>Environment:
  
  
  Request Method: GET
  Request URL: http://whimbee-django.alwaysdata.net/api/v2/eventchat/?format=json
  
  Django Version: 1.4.1
  Python Version: 2.6.6
  Installed Applications:
  ('django.contrib.auth',
   'django.contrib.contenttypes',
   'django.contrib.sessions',
   'django.contrib.sites',
   'django.contrib.messages',
   'django.contrib.staticfiles',
   'django.contrib.admin',
   'django.contrib.admindocs',
   'tastypie',
   'core',
   'iospush')
  Installed Middleware:
  ('django.middleware.common.CommonMiddleware',
   'django.contrib.sessions.middleware.SessionMiddleware',
   'django.middleware.csrf.CsrfViewMiddleware',
   'django.contrib.auth.middleware.AuthenticationMiddleware',
   'django.contrib.messages.middleware.MessageMiddleware',
   'core.middleware.XsSharing')
  
  
  Traceback:
  File "/usr/local/alwaysdata/python/django/1.4.1/django/core/handlers/base.py" in get_response
    111.                         response = callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/alwaysdata/python/django/1.4.1/django/views/decorators/csrf.py" in wrapped_view
    77.         return view_func(*args, **kwargs)
  File "/home/whimbee-django/modules/django_tastypie-0.9.15-py2.6.egg/tastypie/resources.py" in wrapper
    264.                 return self._handle_500(request, e)
  File "/home/whimbee-django/modules/django_tastypie-0.9.15-py2.6.egg/tastypie/resources.py" in _handle_500
    286.             return self.error_response(request, data, response_class=response_class)
  File "/home/whimbee-django/modules/django_tastypie-0.9.15-py2.6.egg/tastypie/resources.py" in error_response
    1243.             serialized = self.serialize(request, errors, desired_format)
  File "/home/whimbee-django/modules/django_tastypie-0.9.15-py2.6.egg/tastypie/resources.py" in serialize
    397.         return self._meta.serializer.serialize(data, format, options)
  File "/home/whimbee-django/modules/django_tastypie-0.9.15-py2.6.egg/tastypie/serializers.py" in serialize
    187.         serialized = getattr(self, "to_%s" % desired_format)(bundle, options)
  File "/home/whimbee-django/modules/django_tastypie-0.9.15-py2.6.egg/tastypie/serializers.py" in to_json
    356.             return simplejson.dumps(data, cls=json.DjangoJSONEncoder, sort_keys=True, ensure_ascii=False)
  File "/usr/languages/python/2.6/lib/python2.6/json/__init__.py" in dumps
    237.         **kw).encode(obj)
  
  Exception Type: TypeError at /api/v2/eventchat/
  Exception Value: __init__() got an unexpected keyword argument 'default'
  </code></pre>