override_method should substitute action

Closes #1106.

A view's action is dependent on the request method. When overriding the method (e.g. to generate a form for a POST request on a GET call to the browseable API), the action should be updated as well. Otherwise, viewset functions may be in a weird limbo state where a 'list' action has a POST method.

Should address https://github.com/tomchristie/django-rest-framework/issues/1106

Not sure where relevant unit tests would be for this -- happy to add though