Allow uppercase methods in action decorator.

Previously, using uppercase for the method argument would silently fail
to route those methods.

rest_framework/routers.py

@@ -136,6 +136,7 @@ class SimpleRouter(BaseRouter):
             attr = getattr(viewset, methodname)
             httpmethods = getattr(attr, 'bind_to_methods', None)
             if httpmethods:
+                httpmethods = [method.lower() for method in httpmethods]
                 dynamic_routes.append((httpmethods, methodname))
 
         ret = []