Extract the content length from an input line.

def _content_length(line):
        """Extract the content length from an input line."""
        if line.startswith(b'Content-Length: '):
            _, value = line.split(b'Content-Length: ')
            value = value.strip()
            try:
                return int(value)
            except ValueError:
                raise ValueError("Invalid Content-Length header: {}".format(value))

        return None