title: mongo-connector gives number_format_exception for geopoint fields
body: <p>When syncing data from mongodb to Elastic Search via mongo-connector, for the properties defined as geo_point in the mapping config, the logs show an error :
  "error":{"type":"number_format_exception","reason":"For input string: \"-71.0, 41.0\""}}}</p>
  
  <p>My mapping config looks like this:</p>
  
  <pre><code>"locP": {
          "properties": {
            "coordinates": {
              "type": "geo_point"
            }
         }
        }
  </code></pre>
  
  <p>I have tried the following:
  Update2 from solution on <a href="https://stackoverflow.com/questions/36772351/does-mongo-connector-supports-adding-fields-before-inserting-to-elasticsearch">Does Mongo-connector supports adding fields before inserting to Elasticsearch?</a> </p>
  
  <pre><code>        if key == 'GeoPoint':
          value = {'lat': value['lat'], 'lon': value['lon']}
  </code></pre>
  
  <p>But still the same error. </p>
  
  <p>If I try the Update1 from the above link, error comes for undefined "geopoint"</p>
  
  <pre><code>elif isinstance(value, Geopoint):
      return self.format.document({'lat': value['lat'], 'lon': value['lon']})
  </code></pre>