Fixed API create_order to allow setting line totals.

$order->add_product expects totals without the `line_` prefix. See https://github.com/woothemes/woocommerce/blob/master/includes/abstracts/abstract-wc-order.php#L149

Without this, setting product coupons doesn't work correctly. (The coupon is saved to the order but no discounts are added to the product.)
Merge pull request #6552 from Aaron3/patch-1

Fixed API create_order to allow setting line totals.