Fix dev server index file (#281)

* Fix dev server index file

When using custom template Asset (like parcel-plugin-pug) with `parcel src/index.pug` it generates correct index.html,
but basename of file is index.pug rather than index.html, so server sends incorrect file.

* Rewrited with using generateBundleName method. Thanks to @devongovett

src/Server.js

@@ -32,7 +32,7 @@ function middleware(bundler) {
     function sendIndex() {
       // If the main asset is an HTML file, serve it
       if (bundler.mainAsset.type === 'html') {
-        req.url = '/' + bundler.mainAsset.basename;
+        req.url = `/${bundler.mainAsset.generateBundleName(true)}`;
         serve(req, res, send404);
       } else {
         send404();