Async jobs don't run when added to a task obtained via find

It's common to obtain a reference to an already created task using the `find` class method.

``` ruby
task = Resque::Plugins::MultiStepTask.find(task_id)
```

If you then try to add a job to that task

``` ruby
task.add_job(SomeJob)
```

the job is never queued: that only happens if the task is `started`, which is tracked by an instance variable that isn't persisted.
