Fix line items and sku

papal express checkout works in 4 steps:

The shop calls SetExpressCheckout on paypal to set up the payment and get a token
redirect the buyer to paypal with the token
After the buyer has agreed to the payment, he is returned back to the shop
The shop calls DoExpressCheckoutPayment to finalize the payment. the user dont see this.
what is happening here is that, we (send item details on the first step)[https://github.com/radar/better_spree_paypal_express/blob/master/app/controllers/spree/paypal_controller.rb#L45], so paypal shows it. but in the last step to finalize it, we (dont send the item details)[https://github.com/radar/better_spree_paypal_express/blob/master/app/models/spree/gateway/pay_pal_express.rb#L38]. so paypal dont care of it in the emails and so on.