Add failing test for updating daily hit total count

We previously made hits counts update if the count has changed since
the previous import (in 389f9d1). We need to do the same for daily_hit_totals
so that the graphs update as well.
Make the update of daily hit totals idempotent

* properly this time
* Ill-advisedly, we've got a column called "count". This causes some problems.
  If you refer to it, use `count` (with backticks). Not doing so means that mySQL
  will read it as the function COUNT (with no parameters). This is probably not
  what you want when operating on this table.
Merge pull request #186 from alphagov/update_daily_hits_total_count

Update count on daily hit totals if hits counts changed