Support JSON values in extras, when returning JSON from API

[seanh]

For example, edit the warandpeace dataset and add an extra with key "comments" and value "[{u'date': u'Tue Aug 21 13:01:48 GMT 2012', u'comment': u'testing ckan', u'userId': u'11702'}, {u'date': u'Wed Aug 22 13:01:48 GMT 2012', u'comment': u'testing ckan again', u'userId': u'11702'}]", now visit API URLs such as:

/api/search/dataset?sort=metadata_modified+desc&all_fields=1&limit=10

/api/rest/dataset/warandpeace

/api/action/package_show?id=warandpeace

The value of the extra comes out as a string in the dataset JSON, rather than having its contents unpacked into a list of JSON objects.