deleteDocument API can fail with obscure error

[cddude229]

new test:

    "Support deleting a doc that doesn't exist" in {
      val delFut = restClient.deleteDocument(index, tpe, new QueryRoot(TermQuery("text7", "here7")))
      Await.result(delFut, 10.seconds) // May not need Await?
    }

Failure mode:

- should Support deleting a doc that doesn't exist *** FAILED ***
  com.sumologic.elasticsearch.restlastic.RestlasticSearchClient$ReturnTypes$ElasticErrorResponse: ElasticsearchError(status=400): JString({"error":{"root_cause":[{"type":"parse_exception","reason":"Failed to derive xcontent"}],"type":"parse_exception","reason":"Failed to derive xcontent"},"status":400})
  at com.sumologic.elasticsearch.restlastic.RestlasticSearchClient$$anonfun$runRawEsRequest$1.apply(RestlasticSearchClient.scala:254)
  at com.sumologic.elasticsearch.restlastic.RestlasticSearchClient$$anonfun$runRawEsRequest$1.apply(RestlasticSearchClient.scala:249)
  at scala.util.Success$$anonfun$map$1.apply(Try.scala:237)
  at scala.util.Try$.apply(Try.scala:192)
  at scala.util.Success.map(Try.scala:237)
  at scala.concurrent.Future$$anonfun$map$1.apply(Future.scala:235)
  at scala.concurrent.Future$$anonfun$map$1.apply(Future.scala:235)
  at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:32)
  at scala.concurrent.impl.ExecutionContextImpl$AdaptedForkJoinTask.exec(ExecutionContextImpl.scala:121)
  at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
  ...

My best guess is it's this bit:

      val documents = Await.result(query(index, tpe, deleteQuery, rawJsonStr = false), 10.seconds).rawSearchResponse.hits.hits.map(_._id)
      bulkDelete(index, tpe, documents.map(Document(_, Map()))).map(res => RawJsonResponse(res.toString))

Looks like we fetch matching documents (0 matches) and then delete them. Some googling (elastic/elasticsearch#8595 (comment)) tells me that the Failure to derive xcontent occurs when the body is empty - so I think that's the issue here.

I'd correct it myself, but I'm not actually sure what to do in the case documents is empty since the return type is Future[RawJsonResponse]

I think this was a regression introduced in #126 btw. cc @CCheSumo