Fix none column retrieve in query.

Previously none column is represented by whole object.
So changed that none column will be represented by null.

When I call this API. 
https://api.usergrid.com/baasio/portal/users?limit=10&ql=select username, email, company

* 2nd data ( user2 ) is not exist company column. 

- expected
```javascript
   "list": [ 
        [ 
            "user1", 
            "user1@baas.io", 
            "BAAS" 
        ], 
        [ 
            "user2", 
            "user2@baas.io", 
            **null**
        ],
...
```
- actually
```javascript
   "list": [ 
        [ 
            "user1", 
            "user1@baas.io", 
            "BAAS" 
        ], 
        [ 
            "user2", 
            "user2@baas.io", 
            { 
                "uuid": "779b8a0f-227b-11e2-9958-02003axxxxx", 
                "type": "user", 
                "name": "user2", 
                "created": 1351592445754, 
                "modified": 1351592445754, 
                "activated": true, 
                "email": "user2@baas.io", 
                "metadata": { 
                    ....
                }, 
                "picture": "http://www.gravatar.com/avatar/68e164df2d02da8360362e7bxxxxxx", 
                "username": "user2" 
            } 
        ],
```