"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.