def test_headers_sent_with_add(
        self, method_type: str, body_type: str | None
    ) -> None:
        """
        Confirm that when adding headers with combine=True that we simply append to the
        most recent value, rather than create a new header line.
        """
        body: None | bytes | io.BytesIO
        if body_type is None:
            body = None
            expected = b"\r\n\r\n"
        elif body_type == "bytes":
            body = b"my-body"
            expected = b"\r\n\r\nmy-body"
        elif body_type == "bytes-io":
            body = io.BytesIO(b"bytes-io-body")
            body.seek(0, 0)
            expected = b"bytes-io-body\r\n0\r\n\r\n"
        else:
            raise ValueError("Unknown body type")

        buffer: bytes = b""

        def socket_handler(listener: socket.socket) -> None:
            nonlocal buffer
            sock = listener.accept()[0]
            sock.settimeout(0)

            while expected not in buffer:
                with contextlib.suppress(BlockingIOError):
                    buffer += sock.recv(65536)

            sock.sendall(
                b"HTTP/1.1 200 OK\r\n"
                b"Server: example.com\r\n"
                b"Content-Length: 0\r\n\r\n"
            )
            sock.close()

        self._start_server(socket_handler)

        headers = HTTPHeaderDict()
        headers.add("A", "1")
        headers.add("C", "3")
        headers.add("B", "2")
        headers.add("B", "3")
        headers.add("A", "4", combine=False)
        headers.add("C", "5", combine=True)
        headers.add("C", "6")

        with HTTPConnectionPool(self.host, self.port, retries=False) as pool:
            r = pool.request(
                method_type,
                "/",
                body=body,
                headers=headers,
            )
            assert r.status == 200
            assert b"A: 1\r\nA: 4\r\nC: 3, 5\r\nC: 6\r\nB: 2\r\nB: 3" in buffer

        Confirm that when adding headers with combine=True that we simply append to the
        most recent value, rather than create a new header line.
        
test/with_dummyserver/test_socketlevel.py