LP1972917: fix circ modifier column in course materials grid

LP1972917: fix circ modifier column in course materials grid

To test:
1. Add a circulation modifier to your system if it doesn't
already have some.
2. In Local Admin > Course Materials > Edit Course > Course
Materials, turn on the circ modifier column in the grid.
3. Associate an item with the course using its barcode.
Make sure you have selected a circulation modifier and
checked the circulation modifier box.
4. Press the "Add Material" button.
5. Note that the circulation modifier column says
"[Object object]"
6. Apply this patch and repeat steps 2-4.
7. Note that the column now has the name of the circulation
modifier.

Signed-off-by: Jane Sandberg <sandbergja@gmail.com>