Serve index.html when in app.asar

Steps:
1. Set directory to be "." (e.g. serve from app.asar directly)
2. Bundle an electron app

Expected:
app://- serves index.html

Actual:
extname(".../app.asar") returns .asar, which fails the guard and serves
file not found.

index.js

@@ -38,8 +38,9 @@ module.exports = options => {
 		const indexPath = path.join(options.directory, 'index.html');
 		const filePath = path.join(options.directory, decodeURIComponent(new URL(request.url).pathname));
 		const resolvedPath = await getPath(filePath);
+		const extname = path.extname(filePath);
 
-		if (resolvedPath || !path.extname(filePath) || path.extname(filePath) === '.html') {
+		if (resolvedPath || !extname || extname === '.html' || extname === '.asar') {
 			callback({
 				path: resolvedPath || indexPath
 			});