Elasticsearch really wants a raw json body and it fails to accept
the request as of es version 5.3

Tested with elasticsearch 5.2 and 5.3.

Before this change 5.2 worked but 5.3 failed with
HTTP/406 "Content-Type header [application/x-www-form-urlencoded] is not supported" [1]

After this change, both worked.

[1] https://phabricator.wikimedia.org/P5158