Content-Type is not set on elasticsearch client requests

Discovered in the course of https://gitlab.com/gitlab-com/infrastructure/issues/2157

The Elasticsearch server expects bulk requests to come with Content-Type: application/x-ndjson but this isn't being set at present.

Reference: https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-bulk.html

Perhaps we're setting the client up wrong, or maybe we need to submit a patch upstream. Worth checking.

I've opened an issue with upstream: https://github.com/olivere/elastic/issues/578