Update compat.py for django 1.7 compatibility

Fix for django 1.7 error:

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py", line 111, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/opt/graphite/webapp/graphite/render/views.py", line 207, in renderView
    response = HttpResponse(content=result, mimetype='application/json')
  File "/opt/graphite/webapp/graphite/compat.py", line 10, in __init__
    super(ContentTypeMixin, self).__init__(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/http/response.py", line 318, in __init__
    super(HttpResponse, self).__init__(*args, **kwargs)
TypeError: __init__() got an unexpected keyword argument 'mimetype'

Based on findings from this thread where "mimetype" has once again been swapped out in favor of "control_type" in 1.7:  https://github.com/caffeinehit/django-oauth2-provider/issues/77