Default end_date in PromotionWeights.bid_history

Makes the behavior match that of PromoteDates.bid_history, which sets the
end_date to today if it's not given as a parameter.

This fixes a bug where the function raised an error if the optional parameter
end_date wasn't provided.

r2/r2/models/bidding.py

@@ -549,6 +549,10 @@ def get_schedule(cls, start_date, end_date, author_id = None):
     def bid_history(cls, start_date, end_date = None, account_id = None):
         from r2.models import Link
         from r2.lib import promote
+
+        if not end_date:
+            end_date = datetime.datetime.now(g.tz)
+
         start_date = to_date(start_date)
         end_date   = to_date(end_date)
         q = cls.query()