add "Insert Downloads" button with media_buttons action instead of media_buttons_context_filter

The `media_buttons_context` filter technically works for adding the "Insert Download" media row button on post and page edit screens. But I noticed nacin recommending the `media_buttons` action here → http://core.trac.wordpress.org/ticket/22559

And you can see a comment explicitly recommend against `media_buttons_context` in `/wp-admin/includes/media.php` → http://core.trac.wordpress.org/changeset/22848

This pull request integrates this very small udpate to the `media_buttons` action.