Support custom headers for SSR pages and API endpoints

There are some minor differences between Netlify and AWS, such as AWS
having support for multi-value headers. next-aws-lambda is made for AWS,
so we need to resolve those differences to make everything work with
Netlify.

We support headers in SSR pages and API endpoints by converting the
first entry in each multi-value header into a plain header.

Netlify also requires all header values to be in string format, so we
take care of that, too.

Author: FinnWoelm

lib/routerTemplate.js

@@ -24,6 +24,34 @@ const getRoute = path => {
   return route || { file: "pages/_error.js" }
 }
 
+// There are some minor differences between Netlify and AWS, such as AWS having
+// support for multi-value headers.
+// next-aws-lambda is made for AWS, so we need to resolve those differences to
+// make everything work with Netlify.
+const callbackHandler = callback => (
+  // The callbackHandler wraps the callback
+  (argument, response) => {
+
+    // Convert multi-value headers to plain headers, because Netlify does not
+    // support multi-value headers.
+    // See: https://github.com/netlify/cli/issues/923
+    response.headers = {}
+    Object.keys(response.multiValueHeaders).forEach(key => {
+      response.headers[key] = response.multiValueHeaders[key][0]
+    })
+    delete response.multiValueHeaders
+
+    // Convert header values to string. Netlify does not support integers as
+    // header values. See: https://github.com/netlify/cli/issues/451
+    Object.keys(response.headers).forEach(key => {
+      response.headers[key] = String(response.headers[key])
+    })
+
+    // Invoke callback
+    callback(argument, response)
+  }
+)
+
 exports.handler = (event, context, callback) => {
   // Get the request URL
   const { path } = event
@@ -50,6 +78,8 @@ exports.handler = (event, context, callback) => {
       multiValueQueryStringParameters: event.queryStringParameters
     },
     context,
-    callback
+    // Wrap the Netlify callback, so that we can resolve differences between
+    // Netlify and AWS (which next-aws-lambda optimizes for)
+    callbackHandler(callback)
   )
 };