helpers.reindex does not keep _timestamp

[JulienPalard]

o/

I use a {"_timestamp": {"enabled": True, "store": True}, mapping to let ES automatically generate timestamp, so my timestamps are typically not in my _source document.

Yet I think getting and reinserting with the correct _timetsamp may be better, if possible.

[honzakral]

Yes, this is a bug that results in lack of implicit support in elasticsearch (it is being worked on in elastic/elasticsearch#8068).

I will close this ticket in favor of #140 where we track this.