Set content-type to application/json

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

Test Plan:
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

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17580