fix default zone argument

Stepping through the code, old and new, zone is set to nil when the call
to ::Time.zone happens in the default argument position, but not when it
happens as the first line of the function.  I don't yet know why.

fixes CNVS-16799

test plan:
- create an assignment with a due date
- view the assignment as a student.  the due date should match

(repeating test plan from commit that caused the regression)
- Create a new course shell and go to the courses settings page and set
  the courses time zone as Eastern and save the change.
- Create a new assignment and put a due date of whatever day you want,
  but have the time be 11pm and save it.
- When the page reloads, hover your mouse over the due date.
- The due date should be correct (on the day after).

Change-Id: I048d766f1552f2c244b5cdebbd7cc3445ed92969
Reviewed-on: https://gerrit.instructure.com/44148
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Reviewed-by: Ethan Vizitei <evizitei@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>