Show a billed fixed-task under itself in the customer_form view

As billed fixed-cost tasks cannot have an active task, the task itself
still is the one that is billed.  Show the task under itself to expose
the fact that it is billed and in which invoice.

stoptime.rb

@@ -820,7 +820,7 @@ module StopTime::Controllers
           if cur_active_task.nil?
             # Apparently there is no active task anymore, probably
             # it was a fixed-cost task
-            @billed_tasks[task] = []
+            @billed_tasks[task] = [task]
           else
             @billed_tasks[cur_active_task] << task
           end