Fix: x-forwarded-host is undefined on Netlify

lib/templates/netlifyFunction.js

@@ -31,6 +31,12 @@ exports.handler = (event, context, callback) => {
     process.env.BINARY_SUPPORT = "yes";
   }
 
+  // x-forwarded-host is undefined on Netlify for proxied apps that need it
+  // fixes https://github.com/netlify/next-on-netlify/issues/46
+  if (!event.multiValueHeaders.hasOwnProperty("x-forwarded-host")) {
+    event.multiValueHeaders["x-forwarded-host"] = [event.headers["host"]];
+  }
+
   // Get the request URL
   const { path } = event;
   console.log("[request]", path);