NullTransport missing the suggest body in the response

[fubuki]

When I use NullTranport.php to test suggest function on elasticsearch 5.6.6 with Elastica 5.3.1, the search method will not return suggest body.

If there are no suggest results in elasticsearch, elasticsearch server will return

{
  "took": 2,
  "timed_out": false,
  "_shards": {
    "total": 15,
    "successful": 15,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": 0,
    "max_score": 0,
    "hits": []
  },
  "suggest": {
    "suggest": [
      {
        "text": "no result",
        "offset": 0,
        "length": 11,
        "options": []
      }
    ]
  }
}

But NullTranport.php will return

{
  "took": 0,
  "timed_out": false,
  "_shards": {
    "total": 0,
    "successful": 0,
    "failed": 0
  },
  "hits": {
    "total": 0,
    "max_score": null,
    "hits": []
  }
}

[ruflin]

The NullTransport is only used for testing and responds with a hardcoded answer: https://github.com/ruflin/Elastica/blob/master/lib/Elastica/Transport/NullTransport.php#L26 We can definitively extend the NullTransport for more tests if needed. Feel free to open a PR.