Fix all day events in calendar feed
Fix all day events in calendar feed

the DTSTART date value included a time which causes outlook
to fail to import it.

closes LS-3207
flag=none

test plan:
  - create an account at outlook.live.com
  - create an assignment in a course due at fancy midnight
  - create an all day event in the calendar
  - create another event with start and end times just for grins
  - make sure your course calendar is checked
  - click the Calendar Feed button
  - click to View Feed in the popup
  - log into your outlook.live.com calendar
  - click on add calendar
  - select Upload from file in the modal's sidebar
  - select the *.ics file canvas exported
  - select a calendar from the dropdown (there's probably only 1)
  - click import
  - close the modal
  > expect the events to be in your calendar

Change-Id: I72e47037a07250bf2edae0d36741c5576fb06278
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/293426
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Jackson Howe <318c10a818c1b1f2ec50cdcb1827b2c6c5ebd7dc@instructure.com>
QA-Review: Jackson Howe <318c10a818c1b1f2ec50cdcb1827b2c6c5ebd7dc@instructure.com>
Product-Review: Ed Schiebel <0ca8a83847a5889cf05cb219c0ab09f6b646243b@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms