Fix for wrong coupon calculation.

There is a missing check wether the item is excluded from discount.

This leads to a wrong calculation of the discount amount here:

https://github.com/magento/magento2/blob/master/app/code/Mage/SalesRule/Model/Validator.php/#L383-384

The reason is, that it calculates the share of the item on the total discount. Since the bug uses the no-discount item to calculate the base items total, the calculated discount is wrong (the no discount item never makes it to the `process()` method, as this takes the `noDiscount` into account).