Add AC::Parameters#to_unsafe_h

As suggested in #16299([1]), this method should be a new public API for
retrieving unfiltered parameters from `ActionController::Parameters`
object, given that `Parameters#to_hash` will no longer work in Rails
5.0+ as we stop inheriting `Parameters` from `Hash`.

[1]: https://github.com/rails/rails/pull/16299#issuecomment-50220919
Merge pull request #18017 from sikachu/4-2-stable-add-params-to_unsafe_h

Add AC::Parameters#to_unsafe_h