'Works like range_for_length; returns None or a ContentRange object
You can use it like::
response.content_range = req.range.content_range(response.content_length)
Though it\'s still up to you to actually serve that content range!'

def content_range(self, length):

    range = self.range_for_length(length)
    if (range is None):
        return None
    return ContentRange(range[0], range[1], length)
