@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 toStringOnSingleHeader() {
        assertEquals("DefaultHttpHeaders[foo: bar]", newDefaultDefaultHttpHeaders()
                .add("foo", "bar")
                .toString());
    }