Removed get_queryset() collisions.

On `djangorestframework (2.3.14)` and `drf-extensions (0.2.5)` I get method collisions when using `DetailSerializerMixin`, because `drf-extensions` is calling this <a href="https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/generics.py#L255">get_queryset()</a>, instead of it's own.
```
Environment:


Request Method: GET
Request URL: http://127.0.0.1:8000/posts/27/

Django Version: 1.7b4
Python Version: 3.4.1
Installed Applications:
('django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'rest_framework',
 'rest_framework.authtoken',
 'grappelli.dashboard',
 'grappelli',
 'django.contrib.admin',
 'social.apps.django_app.default',
 'api')
Installed Middleware:
('api.utils.middleware.DisableCSRF',
 'api.utils.middleware.CustomSessionMiddleware',
 'django.middleware.common.CommonMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.auth.middleware.SessionAuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'django.middleware.clickjacking.XFrameOptionsMiddleware')


Traceback:
File "/Users/maryokhin/.virtualenvs/backend/lib/python3.4/site-packages/django/core/handlers/base.py" in get_response
  111.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/Users/maryokhin/.virtualenvs/backend/lib/python3.4/site-packages/rest_framework/viewsets.py" in view
  78.             return self.dispatch(request, *args, **kwargs)
File "/Users/maryokhin/.virtualenvs/backend/lib/python3.4/site-packages/django/views/decorators/csrf.py" in wrapped_view
  57.         return view_func(*args, **kwargs)
File "/Users/maryokhin/.virtualenvs/backend/lib/python3.4/site-packages/rest_framework/views.py" in dispatch
  400.             response = self.handle_exception(exc)
File "/Users/maryokhin/.virtualenvs/backend/lib/python3.4/site-packages/rest_framework/views.py" in dispatch
  397.             response = handler(request, *args, **kwargs)
File "/Users/maryokhin/.virtualenvs/backend/lib/python3.4/site-packages/rest_framework/mixins.py" in retrieve
  106.         self.object = self.get_object()
File "/Users/maryokhin/.virtualenvs/backend/lib/python3.4/site-packages/rest_framework_extensions/mixins.py" in get_object
  26.             queryset = self.filter_queryset(self.get_queryset(is_for_detail=True))

Exception Type: TypeError at /posts/27/
Exception Value: get_queryset() got an unexpected keyword argument 'is_for_detail'

```