Fix for wrong coupon calculation.

[shredding]

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).

[verklov]

Hello shredding,
Sorry for the delay with the response.
Thank you for your contribution! Our team will review your pull request and respond as soon as our analysis is complete.

[verklov]

Hello shredding,
We were unable to verify this issue on the current Magento 2 code.
We verified two cases of creating Cart Price Rules with coupon and without it:

1. with rule on "Conditions" tab (Applies rules to the whole card if conditions are met)
2. with rule on "Actions" tab (Applies the rule only to cart items matching conditions (leave blank for all items).)
   All cases work properly and the calculation of discount is accurate.