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

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This is the same as #18006 but for `4-2-stable`.