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.
I agree with you, however, as mentioned by you in #143, old style upserts will have invalid state issues.  apply_update will basically add the update_spec (field, values) pair to the target document and will perform upsert and not update as per the changes in lines 100-109 below.

So, instead of limiting connector to be able to work update / upsert seamlessly only when "_source" field is present, we can think of a solution that keeps the new style update with an upsert possibility.