Rest: Add all meta fields to the top level json document.

Some of our meta fields (such as _id, _version, ...) are returned as top-level
properties of the json document, while other properties (_timestamp, _routing,
...) are returned under `fields`. This commit makes all meta fields returned
as top-level properties.

So eg. `GET test/test/1?fields=_timestamp,foo` would now return

```json
{
   "_index": "test",
   "_type": "test",
   "_id": "1",
   "_version": 1,
   "_timestamp": 10000000,
   "found": true,
   "fields": {
     "foo": [ "bar" ]
   }
}
```

while it used to return

```json
{
   "_index": "test",
   "_type": "test",
   "_id": "1",
   "_version": 1,
   "found": true,
   "fields": {
     "_timestamp": 10000000,
     "foo": [ "bar" ]
   }
}
```