gh classroom assignment-grades downloads invalid csv

After calling `gh classroom assignment-grades` for a given assignment, my csv file have assigned grades equal to 0 for all students, despite that they have some other grade which I can see in the assignment view on classroom page.

E.g. my file contains:
```csv
,points_awarded,points_available
,0,4
```
while in the view I see `3.0000000000000004/4`.