Preserve line item field values for rules in commerce_cart_commerce_order_load

In the function commerce_cart_commerce_order_load() currently the whole line item is build new - if its a product line item.
Unfortunately that way all possible field values of the line item are lost and can't be used in the reaction rule commerce_product_calculate_sell_price.
Since it could be possible that some field data are required to calculate the appropriate price the field values should be preserved.
As discussed with Ryan over IRC the patch replaces the function commerce_product_line_item_new by commerce_product_line_item_populate. That way a fresh calculation should be possible as before without loosing any other field data.

I've tested it in my environment and it works for me - but since my environment is a little bit special ;) the patch needs more testing.