Set Content-Type and Accept headers

[jordansissel]

This is necessary to support elastic/elasticsearch#22691 when it lands. Without fixing this, this input will (likely?) not work against new Elasticsearch releases.

[jordansissel]

Blocked on elastic/elasticsearch-ruby#400

[jordansissel]

#56 works around it.

[karmi]

Blocking resolved in elastic/elasticsearch-ruby@76f8679 and released as https://rubygems.org/gems/elasticsearch-transport/versions/5.0.2

[karmi]

Version https://rubygems.org/gems/elasticsearch-transport/versions/5.0.3 released, which addresses a bug in the Manticore adapter when handling options.