"Content-Disposition" is stored directly in the headers array

Since the function now uses `JHttpFactory::getHttp()` instead of `stream_get_meta_data()`, the "Content-Dispositon" data is now stored directly in the `$response->headers['Content-Disposition']` instead of `$response->headers['wrapper_data']['Content-Disposition']`.

Currently in Joomla 3.0.2, you can't install extensions directly from an URL if the URL points to a dynamic created link (like in Akeeba Release System or CTransifex). This is because it doesn't detect a valid filename.

This commit fixes this.