_kuzzle_info.updatedAt and .updater are still null after a mUpdate() operation

[psam44]

The single update() operation is not affected because its code is different.

The bug is at /lib/service/storage/elasticsearch.ts#L3120, _processExtract():

    _source: Object.assign({}, metadata, document.body),

The metadata content is correct, but assign() states that "Later sources' properties overwrite earlier ones.",
and document.body has already these properties, with null values.
I got the expected behaviour with this hack in the distributed js:

            _source: Object.assign({}, document.body, metadata),

I didn't investigate on the two other assign() usages in this file, but they probably have a similar defect.

Context (Environment)

Kuzzle version: master
Node.js version:
SDK version:

[rolljee]

Thank you for reporting this issue, we will have a look into into it.

If you have the time to do it PRs are welcomed