Fix "Get course level assignment" API call
Fix "Get course level assignment" API call

flag = none
Refs OREO-1269

Default proc was still attached in assignments rollup
Change to_h method to hash.default = nil

Test Plan:
* Add assignments to course
* Make sure at least one student is enrolled
** Hit api/v1/courses/{courseid}/analytics/assignments

Change-Id: Idf8cc569a0d033de47644828d0b5f44f97907fb6
Reviewed-on: https://gerrit.instructure.com/c/analytics/+/281804
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Kwok Lam <1915a5c3bcd98ea020140c5ec14d00340a6b5c2a@instructure.com>
Reviewed-by: Bryant Larsen <742f2a8f2b517d3104fd77d6be8a2bacad741bb8@instructure.com>
QA-Review: Kwok Lam <1915a5c3bcd98ea020140c5ec14d00340a6b5c2a@instructure.com>
Product-Review: Noah Rush <8808f9a020cb2154e152e287940d896a1b4241d2@instructure.com>

instructure/analytics,instructure/analytics,instructure/analytics