Order should trigger a sort iff order was changed

In response to https://github.com/jashkenas/backbone/issues/3199 this doesn't make sort fire if the order was unchanged during a `set`.

I'm slightly hesitant about setting `order = false` when `!orderDifferent` but since nothing actually did change wrt the order, it works.
you should be able to get rid of the `order && order.length` part of the conditional here too: `if (toAdd.length || orderChanged) {`