Fix invoice period calculation

Currently, the end time of the period was always the creation time of
the invoice (due to the initialisation), as it is generally by definition
later than any of the task periods.

stoptime.rb

@@ -431,10 +431,9 @@ module StopTime::Models
     # Returns the invoice period based on the contained tasks (Array of Time).
     # See also Task#bill_period.
     def period
-      # FIXME: maybe should be updated_at?
-      p = [created_at, created_at]
-      return p if tasks.empty?
+      return [created_at, created_at] if tasks.empty?
 
+      p = [DateTime.now, DateTime.new(0)]
       tasks.each do |task|
         tp = task.bill_period
         p[0] = tp[0] if tp[0] < p[0]