def to_header(self):
        """Converts the object back into an HTTP header."""
        ranges = []
        for begin, end in self.ranges:
            if end is None:
                ranges.append("%s-" % begin if begin >= 0 else str(begin))
            else:
                ranges.append("%s-%s" % (begin, end - 1))
        return "%s=%s" % (self.units, ",".join(ranges))