[ActionController] Freeze params

One of the many areas of dealing with web apps is interaction with params.

In complex applications often these params are passed into services. When thinking about a concept - params are data send from user browser (or more generally - from a client), and they should never change (if they’re mutating they mean different concept - not params anymore, but some generic data).

I think the best way would be to add such option to config, disabled at first - but with an opportunity to be enabled for curious users.

If these propositions get approval, I will make relevant PR into Rails repository.