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.
sorry, I meant that the first part of the conditional shouldn't be necessary. `order = false` is probably fine, but maybe the later line where the sort event is triggered could check for `orderChanged` instead.