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`.

Empieza reproduciendo `gh classroom assignment-grades` para una tarea cuya vista de Classroom muestra calificaciones distintas de cero; después, inspecciona cómo el comando asigna esas calificaciones al CSV. Se considera terminado cuando `points_awarded` coincide con las calificaciones mostradas en Classroom, en lugar de informar de cero para cada estudiante.

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