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" ]
}
}
```
7c698146f5