Fix response header management in Fastify plugin

[smaye81]

Fastify maintains headers on the reply object and copies to the raw response during reply.send but we are not using reply.send with this plugin. So we need to manually copy them over before handing off to vanilla Node.

This was causing some issues with CORS as it was preventing the Access-Control-Allow-Origin header from being set on the response. fastify/cors sets it on the reply headers, but this was never making it over to the raw response.