Different behaviour between `Spree::Api::LineItemsController#create` and `#update`

The create method uses `:as => :api` but the `update` does not in [`Spree::Api::LineItemsController`](https://github.com/spree/spree/blob/master/api/app/controllers/spree/api/line_items_controller.rb#L19)

Is this intended? It looks weird to me, but there may be some reason I don't see. If that's unintended, I'd gladly file a pull request.

Cheers and thanks for Spree!