[Consistency] security:searchUsers method should return the same response than the one from the API

[Njuelle]

Expected Behavior

When calling the security:searchUsers method with the SDK, the response must be consistent with the API :

{
  "status": 200,
  "error": null,
  "action": "searchUsers",
  "controller": "security",
  "requestId": "<unique request identifier>",
  "result": {
    "total": 2,
    "hits": [
      {
        "_id": "kuid1",
        "_source": {
          // User content
        }
      },
      {
        "_id": "kuid2",
        "_source" {
          // User content
        }
      }
    ]
  }
}

Current Behavior

Currently the SDK returns a user's information in the content object instead of _source :

{
    aggregations: undefined,
    hits:
      [ User { _kuzzle: [Kuzzle], _id: 'user2', content: [Object] },
        User { _kuzzle: [Kuzzle], _id: 'user1', content: [Object] },
        User { _kuzzle: [Kuzzle], _id: 'user3', content: [Object] } ],
    fetched: 3,
    total: 3 
}

Possible Solution

Add the _source object to the response and keep the content object as a reference of the first one to avoid breaking change.
The content object should be deprecated.

Bonus : Throw a warning only in dev mode to prevent that the content object is deprecated