@Override
    public HttpHeaders add(HttpHeaders headers) {
        if (headers instanceof DefaultHttpHeaders) {
            this.headers.add(((DefaultHttpHeaders) headers).headers);
            return this;
        } else {
            return super.add(headers);
        }
    }
@Test
    public void nullHeaderNameNotAllowed() {
        assertThrows(IllegalArgumentException.class, new Executable() {
            @Override
            public void execute() {
                new DefaultHttpHeaders().add(null, "foo");
            }
        });
    }