[wip]default_package_show_schema resources can cause 500 error.

edit:The current [default_show_package_schema](ckan/blob/master/ckan/logic/schema.py#L219) adds some fields so they don't get stripped if they are missing to the resource subschema, but at the moment the empty validators inject a missing object in their place. But Missing is not jsonable and causes 500s in the [package](https://github.com/ckan/ckan/blob/master/ckan/controllers/package.py#L510), [related](https://github.com/ckan/ckan/blob/master/ckan/controllers/related.py#L120) and in the api controller

checkout pdeu staging and click on related for a live example
http://www.pdeu.staging.ckanhosted.com/dataset/01-bve-crebo--beroepsopleidingen-ministerie-van-ocw-2010-2011-ministerie-van-ocw