public void add(String name, String value) {
        headers.computeIfAbsent(name, k -> new ArrayList<>(1)).add(value);
    }
@Test
    public void testAdd() {
        HttpHeaders headers = new HttpHeaders(2);
        headers.set("Connection", "Keep-Alive");
        headers.add("Connection", "close");

        Assert.assertEquals("Keep-Alive", headers.get("Connection"));
        Assert.assertEquals(Arrays.asList("Keep-Alive", "close"), headers.getValues("Connection"));
    }