param as is not work

```ruby
params do
    optional :page_size, as: :per_page, type: Integer
end

params #=> {page_size: 10} not have per_page
declared(params) #=> {per_page: 10}
```

i upgrade from 1.31 to 1.6.0


@brynary @tmornini @ctennis @olleolleolle @bernd 
How should I write？ thanks