Use HTTP POST for elasticsearch.scroll().

[gmazzola]

Addresses Issue #971

This pull request POSTs a JSON payload to the Elasticsearch scroll endpoint. We generate the POST body sanely by prioritizing the body argument first, and if it is absent, we generate a valid body using the scroll_id and scroll arguments.

This PR does not change the method signature. I tested it against an Elasticsearch 6.2.4 cluster and also via the existing unittests. I have also signed the contributor agreement. :)

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[ghost]

Hi @gmazzola, we have found your signature in our records, but it seems like you have signed with a different e-mail than the one used in your Git commit. Can you please add both of these e-mails into your Github profile (they can be hidden), so we can match your e-mails to your Github profile?

[gmazzola]

Will re-open this pull request using my personal git credentials. Sorry!