Make sure APIRoot.get can take on args, kwargs so router can be embedded...

... within any URL pattern.

I was dealing with a simple project where my router was being included in a URL pattern with a named group which resulted in APIRoot view error `get() got an unexpected keyword argument '<name of group>'`. Digging into the code, I think a regression was introduced with the https://github.com/tomchristie/django-rest-framework/commit/aff88d15f7a483bca2da120339b1b346aa8b1d4c where earlier `APIRoot.get()` was able to take on additional args and kwargs because of the `@api_view` decorator, now it is not. 

Simple steps to reproduce error on `master` branch: 
* In your `urls.py` instead of using `url(r'^somenamespace/', include(router.urls))` use `url(r'^(?P<namedgroup>[-\w]+)/', include(router.urls))`

Please let me know if I'm understanding it wrong, or haven't patched a proper fix. 
You'd need to change this slightly so that `format` is not included, but simply accessed as `kwargs.get('format', None)` otherwise it's positioning would conflict with any unnamed `args`.