Add support for res.redirect in API routes

Return the res object from writeHead, so that other methods (like end())
can be chained. We may need to adjust other methods on our res object
as well to match the http ServerResponse that Next.js expects.

Author: FinnWoelm

lib/templates/createResponseObject.js

@@ -24,6 +24,10 @@ const createResponseObject = ({ onResEnd }) => {
   res.writeHead = (status, headers) => {
     response.statusCode = status;
     if (headers) res.headers = Object.assign(res.headers, headers);
+
+    // Return res object to allow for chaining
+    // Fixes: https://github.com/netlify/next-on-netlify/pull/74
+    return res;
   };
   res.write = (chunk) => {
     if (!response.body) {