elasticsearch_http: Use source._id automatically

This is an alternative to using document_id option.  Better suited to diverse deployments when some sources provide unique id and others don't.
Thanks, I see. Both proposed solutions (conditional document_id in config file, sprintf patch) would work too, but IMO these are more complex than the proposed change (and changing sprintf return value might have side effects).  I still think that defaulting to a copy of _id from the document is the obvious choice here, because if document contains _id, Elasticsearch errs out when _id from the index metadata (or the autogenerated one) is different:
```
POST /_bulk HTTP/1.1
...
{"create":{"_index":"foo","_type":"bar"}}
{"_id":"1"}

HTTP/1.1 200 OK
...
{"took":29,"errors":true,"items":[{"create":{"_index":"foo","_type":"bar","_id":"hZJ9tiCwRNC54-pFuVrLgw","status":400,"error":"MapperParsingException[failed to parse [_id]]; nested: MapperParsingException[Provided id [hZJ9tiCwRNC54-pFuVrLgw] does not match the content one [1]]; "}}]}^C
```