You can not create a dataset private by default

The schema used when creating a dataset (`default_create_package_schema`) does not include `private` so if you have this property set to True in your dict it is ignored. On the frontend this does not come up because we are doing the stage thing, which actually uses `package_update` on the 3rd stage, where the visibility is set (The default update schema does consider `private`).

The problem occurs eg when creating a package calling `get_action('package_create')` or via the API.