Refactor ActionDispatch::Http::Parameters#normalize_encode_params

There was an unneeded `case` branch at `ActionDispatch::Http::Parameters#normalize_encode_params`. While removing it, I have decided that an `if/else` will work a bit better than:

```ruby
case val
when Array
  # Do stuff...
else
  # Do some other stuff...
end
```

Additionally there seem to be a `TODO` note in the comment above the function and I am not sure if it is still up-to-date. Anyway, I decided not to go that deep, into what seemed as a simple refactor.