override_method should substitute action

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.
Fixed action_map being pulled from wrong object
Merge pull request #1763 from fongandrew/patch-1

override_method should substitute action