startDate and endDate do not save in UTC format on reserve.json

An item on reserve for a course has an associated start date and end date. It inherits the start date and end date from the term. If the startDate and endDate have not been explicitly set, than they do not exist on the item. If they are explicitly set, they do not always save in UTC format.

Steps to reproduce on folio-snapshot:

1. Log into folio-snapshot with a user with appropriate permissions
2. Select a course, and add an item to the course. Verify that the term fields show up on the item and match the term on the course.
3. Click the pencil to edit the reserve item.
4. Modify the start date or end date on the reserve.
5. Save the reserve and then reopen it in edit mode.
6. Use developer tools to look at the underlying JSON.

Expected behavior - startDate and endDate are both visible in the underlying JSON in UTC format.
Actual behavior - startDate and endDate are stored. The attribute(s) that was/were modified is stored in non-UTC format. If an attribute did not previously exist, and is not set when saved, the attribute is created and stored in UTC format.