Improve add line item to order

[DanielePalombo]

The add method in Spree::OrderContents accepts three parameters but it's called without the third in https://github.com/solidusio/solidus/blob/master/frontend/app/controllers/spree/orders_controller.rb#L55 .

I think we can pass this as method.
In this way who wants to change the add feature doesn't need to override the populate action, but can change logic inside Spree::OrderContents after modify that method.

ex:

def add_option_params
  {}
end

def create
  ...
  begin
    @line_item = @order.contents.add(variant, quantity, add_option_params)
  rescue ActiveRecord::RecordInvalid => e
    @order.errors.add(:base, e.record.errors.full_messages.join(", "))
  end
  ...
end

The same logic can be applied on
https://github.com/solidusio/solidus/blob/master/api/app/controllers/spree/api/line_items_controller.rb#L15