Serve 404 page with status code 404, not 200

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

.eleventy.js

@@ -89,6 +89,7 @@ module.exports = function(eleventyConfig) {
 
         browserSync.addMiddleware("*", (req, res) => {
           // Provides the 404 content without redirect.
+          res.writeHead(404, {"Content-Type": "text/html; charset=UTF-8"});
           res.write(content_404);
           res.end();
         });