Introduce RangesAtEndpoint.unwrap to simplify StreamSession.addTransferRanges

Arguably, since this is only performed in one place, we could leave it in addTransferRanges, but it should be a helper method anyway, and given unwrap() is a feature of Range, we should implement that in RangesAtEndpoint IMO. I have introduced this method, which avoids allocating a new collection unnecessarily, corroborates we have at most one wrap-around range, and introduced unit tests for the method.