'Converts the object into `Content-Range` HTTP header,
based on given length'

def to_content_range_header(self, length):

    range_for_length = self.range_for_length(length)
    if (range_for_length is not None):
        return ('%s    %d-%d/%d' % (self.units, range_for_length[0], (range_for_length[1] - 1), length))
    return None
