Railsのレスポンスに任意のHTTPヘッダを追加するRackミドルウェア

config/initializers/additional_http_headers.rb

class AdditionalHTTPHeaders
  def initialize(app)
    @app = app
  end

  def call(env)
    status, headers, body = @app.call(env)
    additional_headers.each{|k, v| headers[k] = v }
    [status, headers, body]
  end

  private
    def additional_headers
      {
        'x-foo' => 'bar',
      }
    end
end

Rails.application.config.middleware.insert_before(
  ActionDispatch::Cookies, AdditionalHTTPHeaders
)