AjaxResponse::setCacheDuration - pass zero to disable CDN caching

Before this change calls to ``AjaxResponse::setCacheDuration`` with value of zero passed did not work as expected - CDN caching was not disabled.

Emit the following HTTP response headers:

```
Cache-Control: s-maxage=0 must-revalidate, max-age=0
```

instead of

```
Expires: Mon, 26 Jul 1997 05:00:00 GMT
Cache-Control: no-cache, must-revalidate
Pragma: no-cache
```

@michalroszka / @prein / @adamkarminski / @Wikia/platform-team