Add support for multivalue headers.

Author: richtera

lib/serve.js

@@ -36,6 +36,10 @@ function createCallback(response) {
     for (const key in lambdaResponse.headers) {
       response.setHeader(key, lambdaResponse.headers[key]);
     }
+    for (const key in lambdaResponse.multiValueHeaders) {
+      const items = lambdaResponse.multiValueHeaders[key];
+      response.setHeader(key, items);
+    }
 
     if (lambdaResponse.body) {
       response.write(