Setting the content_type sets the response Content-Type

Otherwise we have two headers, 'Content-Type' and 'CONTENT_TYPE'.
I don't know when Rails decides to use one or the other.

Author: bf4

lib/active_model_serializers/register_jsonapi_renderer.rb

@@ -57,7 +57,6 @@ def serialize_jsonapi(json, options)
 ActionController::Renderers.add :jsonapi do |json, options|
   json = serialize_jsonapi(json, options).to_json(options) unless json.is_a?(String)
   self.content_type ||= media_type
-  headers.merge! ActiveModelSerializers::Jsonapi::HEADERS[:response]
   self.response_body = json
 end