Fix calculation of base total in magento report with discount

[kpitn]

Actually magento store discount with negative value : - 10

If you have an order with subtotal = 90 and base_discount_invoiced = -10

When Magento do base_subtotal_invoiced - base_discount_invoiced => 90 -- 10 => 90 +10 => 100

[tmotyl]

@kpitn Can you please provide steps to reproduce the issue, so we can test the patch during review?

[kpitn]

1. Make an order with some coupon with discount amount.
2. In admin goes to > Report > Sales > Coupon
3. Display your order : with the correct date / order status

You will see the last column name "Total amount" with incorect values.

[colinmollenhour]

Would + not be more correct? E.g. if there was somehow a negative price rule? Googling around I see several people trying to do this by changing the validation class names to allow negative values.

[kpitn]

Actually magento use ABS() for other sum (look in the main file)

[tbaden]

@LeeSaferite @Flyingmana @drobinson
seems ready to merge