Course grade miscalculates when you excuse a grade, if using Categories & Weights

For Gradebooks that use Categories & Weights, the overall Course Grade is miscalculated if you excuse a grade.

(If not using Cats & Weights, course grade calculates correctly when you excuse a grade.)

I replicated on https://qa1-us.nightly.sakaiproject.org/ and on https://trunk-mysql.nightly.sakaiproject.org

Steps to replicate:

1. Set up a Gradebook with Categories & Weights, such as:
     - Homework = 40%
     - Exams = 60%
   2.Create a few items in the Homework category, such as:
     - HW1 = 100 pts
     - HW2 = 100 pts
     - HW3 = 100 pts
   3. Just to be thorough, create one or more items in the Exams category, such as:
     - Exam 1 = 100 pts
   4. Assign a student the following grades:
     - HW1 = 100 pts
     - HW2 = 80 pts
     - HW3 = 60 pts
   5. Notice that the Course Grade is 80%, which is correct.
   6. Excuse the student's grade for HW3 (which has a score of 60), by clicking the dropdown arrow in the cell and choosing "Excuse/Include Grade". The Course Grade doesn't immmediately update – click GRADEBOOK at the top to refresh the page to see the updated calculation.
   7. Notice that the Course grade is now 60%, which is incorrect – it should be 90%.
   8. Notice that the Weighted Average Column for the Homework category displays 90%, which is correct. (Group by Category must be ON to see this.)
   9. Enter a grade for Exam 1, such as 100. Notice that the Course Grade is now 84%, which is incorrect. It should be 96%.

Note: If you go into Settings, and change to "Categories only" or "No categories", you will see that the Course Grade is correctly calculated.

MISCALCULATION EXPLAINED:
When you excuse a grade, two different weighted averages are being calculated for the category. The one displayed in the category's Weighted Average column is correct – but a different, miscalculated average is used in the Course Grade calculation. This miscalculated average wrongly includes the point value for the excused item in the denominator, like so:
(HW1 grade + HW2 grade) / (HW1 pt value + HW2 pt value + HW3 pt value)

The correct formula should be:
(HW1 grade + HW2 grade) / (HW1 pt value + HW2 pt value)