fixes bug 960093 - report list graph & table unchanged by date range

@rhelmer r?

The key ingredient in this patch is that it now passed the `start_date` and `end_date` to the middleware at all. 
Also, when doing so it makes sure it sends it as a `datetime.date`. 
Internally in `socorro/external/postgresq/crashes.py` it converts that to a `datetime.datetime` but that's fine because it becomes `00:00:00+00`.