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 Even if "_source" is available, connector is using "es.index" instead of "es.update" which still overwrites/resets other fields that are not part of update fieldspec.  I have been running these changes on a setup that performs 1000 updates/sec with "_source" disabled.  Works well for me here.  I would really love to see mongo-connector be able to provide such a solution.  Let me know if I need to improve the patch.