fix bug introduced by last commit and add test for it

also refactor check_for_next_todo a bit to depend less on globals

test/fixtures/todos.yml

@@ -230,3 +230,17 @@ end
   completed_at: ~
   show_from: <%= next_week %>
   user_id: 2
+
+18:
+  id: 18
+  user_id: 1
+  context_id: 1
+  project_id: 2
+  description: Call Bill Gates every day
+  notes: ~
+  state: active
+  created_at: <%= last_week %>
+  due: <%= last_week %>
+  completed_at: ~
+  show_from: ~
+  recurring_todo_id: 1