Validator should better respond to `params` for backward compat

684651d264069f01066de9946cc23079583934dd replaced `params` accessor into `controller` in BaseValidator and Validator, then added `params` method back to only BaseValidator.

This caused

    NameError (undefined local variable or method `params' for #<WeakParameters::Validator:0x007fc04af9b020>)

in our existing code.