jobcomp/elasticsearch - append Content-Type to the HTTP header.

The ability to enforce strict content-type checking has existed since
Elasticsearch 5.3 via the http.content_type.required configuration setting.
In 5.x it is optional, and defaults to false, in Elasticsearch 6.0, that
setting defaults to true, and there is no way to disable it.

If required and not appended, Elasticsearch would response with status 406,
so "Content-Type: application/json" is appended from now on. Tested against
Elasticsearch 5.5.1 and 6.1.1.

Bug reported to slurm-users list from E.S. Rosenberg. Patch suggested didn't
check the return code of curl_slist_append() and didn't free the slist.