Fix default date for transaction creation

Until now, the default date was the one the app was started on as the
value was calculated on class initialisation.

web/blueprints/finance/forms.py

@@ -150,7 +150,7 @@ class TransactionCreateForm(Form):
     description = TextField("Beschreibung", validators=[DataRequired()])
     valid_on = DateField(
         "Gültig ab", validators=[Optional()], today_btn=True,
-        today_highlight=True, default=datetime.date.today())
+        today_highlight=True, default=datetime.date.today)
     splits = FieldList(
         FormField(SplitCreateForm),
         validators=[DataRequired()],