Add support for res.redirect in API routes

Copy over the files from the next-aws-lambda package and manually
bundle them into our Netlify Functions. This gives us more flexibility
to customize the compatibility layer between Netlify Functions and
Next.js. For now, no changes have been made to the next-aws-lambda
files and they have been copied as-is.

next-aws-lambda source: https://github.com/serverless-nextjs/serverless-next.js/tree/master/packages/compat-layers/apigw-lambda-compat

Author: FinnWoelm

cypress/fixtures/pages/api/redirect.js

@@ -0,0 +1,6 @@
+export default async function preview(req, res) {
+  const { query } = req;
+  const { to } = query;
+
+  res.redirect(`/shows/${to}`);
+}

cypress/integration/default_spec.js

@@ -526,6 +526,14 @@ describe("API endpoint", () => {
       );
     });
   });
+
+  it("redirects with res.redirect", () => {
+    cy.visit("/api/redirect?to=999");
+
+    cy.url().should("include", "/shows/999");
+    cy.get("h1").should("contain", "Show #999");
+    cy.get("p").should("contain", "Flash Gordon");
+  });
 });
 
 describe("Preview Mode", () => {

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) {