title: mapping in create index in elasticsearch through mongodb river is not taking effect
body: <p>I am trying to index mongodb in elasticsearch using mongodb-river using the following command but the document mapping is not taking effect. It is still using the default analyzer(standard) for field <code>text</code></p>
  
  <p><a href="https://github.com/richardwilly98/elasticsearch-river-mongodb/" rel="noreferrer">Mongodb-river</a>
  The document specifies the creation of index but there is no documentation on how to provide custom mapping. This is what I tried. Is there any other documentation where I can find how to specify custom analyzers etc in using mongodb-river.</p>
  
  <pre><code>curl -XPUT "localhost:9200/_river/autocompleteindex/_meta" -d '
  {
      "type": "mongodb",
      "mongodb": {
          "host": "rahulg-dc",
          "port": "27017",
          "db": "qna",
          "collection": "autocomplete_questions"
      },
      "index": {
          "name": "autocompleteindex",
          "type": "autocomplete_questions",
          "analysis" : {
                  "analyzer" : {
                       "str_search_analyzer" : {
                            "tokenizer" : "keyword",
                            "filter" : ["lowercase"]
                        },
  
                        "str_index_analyzer" : {
                           "tokenizer" : "keyword",
                           "filter" : ["lowercase", "ngram"]
                      }
                  },
                  "filter" : {
                      "ngram" : {
                          "type" : "ngram",
                          "min_gram" : 2,
                          "max_gram" : 20
                      }
                  }
              }
      },
      "autocompleteindex": {
         "_boost" : {
              "name" : "po", 
              "null_value" : 1.0
         },
         "properties": {
                  "po": {
                      "type": "double"
                  },
                  "text": {
                      "type": "string",
                      "boost": 3.0,
                      "search_analyzer" : "str_search_analyzer",
                      "index_analyzer" : "str_index_analyzer"
                  }           
         }
      }
  }'
  </code></pre>
  
  <p>The query returns proper results is I search by full words but does not match any substring match. Also, the boost factor is not showing its effect. </p>
  
  <p>What am I doing wrong ??</p>