Set a cookie in the response headers.

        The arguments are the standard cookie morsels and this is a
        wrapper around the stdlib SimpleCookie code.

def set_cookie(  # type: ignore
            self,
            key: str,
            value: AnyStr='',
            max_age: Optional[Union[int, timedelta]]=None,
            expires: Optional[datetime]=None,
            path: str='/',
            domain: Optional[str]=None,
            secure: bool=False,
            httponly: bool=False,
    ) -> None:
        """Set a cookie in the response headers.

        The arguments are the standard cookie morsels and this is a
        wrapper around the stdlib SimpleCookie code.
        """
        if isinstance(value, bytes):
            value = value.decode()  # type: ignore
        cookie = create_cookie(key, value, max_age, expires, path, domain, secure, httponly)  # type: ignore  # noqa: E501
        self.headers.add('Set-Cookie', cookie.output(header=''))