Fix sending of performance platform

The pp client converts to UTC using the convert_utc_to_bst notify util.

This requires a datatime not a date, pass it a datetime, and add an
assertion in an existing test.

I didn't want to use the midnight conversion util in the test.

Signed-off-by: Toby Lorne <toby.lornewelch-richards@digital.cabinet-office.gov.uk>

app/performance_platform/total_sent_notifications.py

@@ -1,5 +1,6 @@
 from app import performance_platform_client
 from app.dao.fact_notification_status_dao import get_total_sent_notifications_for_day_and_type
+from app.utils import get_london_midnight_in_utc
 
 
 def send_total_notifications_sent_for_day_stats(date, notification_type, count):
@@ -19,8 +20,10 @@ def get_total_sent_notifications_for_day(day):
     sms_count = get_total_sent_notifications_for_day_and_type(day, 'sms')
     letter_count = get_total_sent_notifications_for_day_and_type(day, 'letter')
 
+    start_date = get_london_midnight_in_utc(day)
+
     return {
-        "start_date": day,
+        "start_date": start_date,
         "email": {
             "count": email_count
         },