Do not permanently overwrite Parameters#[]

When stubbing the `ActionController::Parameters#[]` method, do not
permanently overwrite it. Restore the method after verifying simulating
the controller action.

Also renames `#stubbed_model_attributes` from past tense to present `#stub_model_attributes` to reflect action being taken.

This aims to resolve issues mentioned here - https://github.com/thoughtbot/shoulda-matchers/commit/723b5d3552689481e8c6e4df3618aedcc32c0f38#commitcomment-5918371