Handle setups where "_source" is disabled.  Update resets values of fields not present in the update-field-list.

Use es.update instead of es.index when performing an update as the former allows partial update of documents whereas the later resets/discards values from the index for the fields that are not part of the update field set, thereby making such updated documents unsearchable using any previously searchable query terms or filter.
@lovett89 Moreover, the need for a GET is because of using es.index, which is not giving expected performance when the application is doing lots of updates than inserts.  If es.update is used, we can avoid the need for a GET, which would increase the performance of mongo-connector's update method.