Don't mutate the original object when using ActionController::Parameters#fetch

The `ActionController::Parameters#fetch` doesn't work in the same way that Hash#fetch and mutates the original object, what may cause problems in some applications. This PR defines `ActionController::Parameters#convert_hashes_to_parameters!` having the same past behaviour but redefines `ActionController::Parameters#convert_hashes_to_parameters` to not mutate the original object.