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.
Use sentence case (uppercase the first letter), add a space at the beginning of the sentence after the `//`, and add a period at the end per our convention please.