fix clear hold shelf with 25 or more holds

When there are 25 or more shelf expired holds to be processed, the API call
returns an array of objects instead of an object, which the staff client wasn't
prepared to accommodate. This fixes that.

Signed-off-by: Jason Etheridge <jason@esilibrary.com>
Signed-off-by: Mike Rylander <mrylander@gmail.com>