[bugfix] avoid TypeError when resolving enqueued write requests from cache

My scenario:

1. Issue a $save on a resource and bind some promise callbacks to it (by `.then` or `.catch`)
2. Reopen browser (so resoure will be resaved from cache)
3. Try to $save again manually

This will make this error:

> Uncaught TypeError: Cannot read property '$promise' of undefined

This fixes it.