PUT request to update recurrence returns 500 Internal Server Error

## Steps to reproduce
1) Make a `PUT` request to `/events/<event_id>/<rec_id>` with some JSON payload, e.g.
```json
PUT <dev_url>/events/5acc3e9adb9fbc000b2b0d72/2018-05-17
{
   "location": "MH Library UL",
   "rec_id": "2018-05-17T00:00:00",
   "sid": "5acc3e9adb9fbc000b2b0d72"
}
```
2) Server returns
```json
{
  "message": "Internal Server Error"
}
```