Add _source to ReindexOnServer API source settings

[russcam]

ReindexOnServer is missing the ability to specify individual _source fields from the source as per the example in the documentation.:

POST _reindex
{
  "source": {
    "index": "twitter",
    "_source": ["user", "_doc"]  // <-- missing
  },
  "dest": {
    "index": "new_twitter"
  }
}