di add content-type assertion

Author: p

spec/datadog/di/integration/probe_notifier_worker_spec.rb

@@ -46,6 +46,7 @@
     end
 
     http_server.mount_proc('/debugger/v1/input') do |req, res|
+      expect(req.content_type).to eq('application/json')
       payload = JSON.parse(req.body)
       input_payloads << payload
     end