Not all quizzes have a due date

If there is a due date, parse it, otherwise set the due date to None.
Merge pull request #3 from uw-it-aca/bug/quiz_due_date

Not all quizzes have a due date
Adding self._sis_id for more urls
Merge pull request #4 from uw-it-aca/bug/sis_urls_coverage

Adding self._sis_id for more urls
Missed the analytics in the first commit
Merge pull request #5 from uw-it-aca/bug/sis_urls_coverage

Missed the analytics in the first commit
due at can be None

This checks not only for the existance of the due at field, but that it has a real value.
Merge pull request #6 from uw-it-aca/bug/assignment_due_date

due at can be None
Fixes a bug where headers aren't stored in cache.

Headers in the timed cache implementation were being set to [], instead of the actual headers.  Broke pagination in cached canvas resources.
Merge pull request #7 from uw-it-aca/bug/cache_headers

Fixes a bug where headers aren't stored in cache.
Merge pull request #8 from uw-it-aca/develop

Develop