Do not pass down 'ActionController::Parameters' to outside the controller

This deals with regressions similar to #3157. This way the models will always receive a `HashWithIndifferentAccess` instead of a `ActionController::Parameters`. 

One alternative would be to change the `resource_params` helper to always return a plain Hash, but I don't know if we should change the return type of that method and even if we should have it around since most of the parameters manipulation is currently on the parameters sanitizer.

/cc @rafaelfranca @carlosantoniodasilva @josevalim