Test what happens when _week_start_at is not Monday

  (This will blow up)
Blow up if _week_start_at is not on a Monday.

Changing this so that is fails loudly instead of silently,
not going to fix this now because I'm not sure if the implementation
of padding out responses is correct.
Add bad _week_start_at date to error message
Merge pull request #37 from alphagov/fix_period_and_group_by

Fix period and group by