Fix response header management in Fastify plugin

packages/connect-fastify/src/fastify-connect-plugin.ts

@@ -87,6 +87,17 @@ export function fastifyConnectPlugin(
             req.body as JsonValue | undefined
           )
         );
+        // Fastify maintains response headers both on the reply object and on the raw response.
+        // So before we hand off to vanilla node, we need to make sure the headers from
+        // reply are copied over to the raw response.  reply.send does this automatically, but
+        // we are not using reply.send with this plugin.
+        // Note that we are intentionally making reply headers take precedence to adhere to
+        // Fastify's contract:  https://www.fastify.io/docs/latest/Reference/Reply/#getheaders
+        for (const [key, value] of Object.entries(reply.getHeaders())) {
+          if (value !== undefined) {
+            reply.raw.setHeader(key, value);
+          }
+        }
         await universalResponseToNodeResponse(uRes, reply.raw);
       }
     );