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

src/applications/search/fulltextstorage/PhabricatorElasticFulltextStorageEngine.php

@@ -551,6 +551,8 @@ class PhabricatorElasticFulltextStorageEngine
     $uri = $host->getURI($path);
     $data = phutil_json_encode($data);
     $future = new HTTPSFuture($uri, $data);
+    $future->addHeader('Content-Type', 'application/json');
+
     if ($method != 'GET') {
       $future->setMethod($method);
     }