title: Django Rest Framework not finding throttle scope?
body: <p>Extrapolating from the example found in <a href="http://www.django-rest-framework.org/api-guide/throttling/" rel="nofollow">http://www.django-rest-framework.org/api-guide/throttling/</a>, I added the following settings to my DRF setup:</p>
  
  <pre><code>REST_FRAMEWORK = {
  'DEFAULT_THROTTLE_CLASSES': (
      'rest_framework.throttling.AnonRateThrottle',
      'rest_framework.throttling.UserRateThrottle',
      'project.api.throttles.AppEventRateThrottle',
  ),
  
  'DEFAULT_THROTTLE_RATES': { # 86,400 seconds in a day
      'app_events': '10000/day',
      'anon': '10000/day',
      'user': '10000/day',
  },
  'DEFAULT_AUTHENTICATION_CLASSES': (
      'rest_framework.authentication.BasicAuthentication',
      'rest_framework.authentication.TokenAuthentication',
  ),
  'EXCEPTION_HANDLER': 'project.api.exception_handler.custom_exception_handler',
  }
  </code></pre>
  
  <p>This is the simple <code>AppEventRateThrottle</code> class, located in <code>project.api.throttles</code></p>
  
  <pre><code>from rest_framework.throttling import AnonRateThrottle
  
  class AppEventRateThrottle(AnonRateThrottle):
       scope = 'app_events'
  </code></pre>
  
  <p>The simple function-based API view that I'm trying to throttle:</p>
  
  <pre><code>from project.api.throttles import AppEventRateThrottle
  @api_view(['POST'])
  @throttle_classes([AppEventRateThrottle])
  def grouped_event_create(request):
      return Response("Hello, world!")
  </code></pre>
  
  <p>However, every time I make this API call, I get</p>
  
  <pre><code>  File "/usr/local/lib/python3.4/dist-packages/rest_framework/throttling.py", line 94, in get_rate
  raise ImproperlyConfigured(msg)
   django.core.exceptions.ImproperlyConfigured: No default throttle rate set for 'app_events' scope
  </code></pre>
  
  <p>It seems it cannot find the 'app_events' key in the <code>DEFAULT_THROTTLE_RATES</code> dictionary, but it's clearly defined.</p>