Fixed Safari bug with abort method.

In Safari there is bug, when you use `removeAfterUpload`. At `item.complete` uploader tries to remove item and `remove()` method calls upload to remove `this` from queue. Then in `removeFromQueue` there is `item.cancel()` which will abort xhr request and unfortunatelly even that it is aborted it will trigger `onabort` event. And this event will triger `item.complete` so we have classic loop here :)

I fixed it in an easiest way, maybe you have better solution.