Update ft_billing with real postage data

* Changed update_fact_billing DAO function to update the table with the
real data for postage instead of hard-coding in 'second'.
* Added a test for the create nightly billing task to test that rows
with different postage are being inserted correctly.

app/dao/fact_billing_dao.py

@@ -237,7 +237,7 @@ def update_fact_billing(data, process_day):
                     process_day,
                     data.crown,
                     data.letter_page_count,
-                    "second")
+                    data.postage)
     billing_record = create_billing_record(data, rate, process_day)
     table = FactBilling.__table__
     '''