fix deprecated buffer creation

Author: thornjad

lib/core/index.js

@@ -55,7 +55,7 @@ function shouldCompressBrotli(req) {
 
 function hasGzipId12(gzipped, cb) {
   const stream = fs.createReadStream(gzipped, { start: 0, end: 1 });
-  let buffer = Buffer('');
+  let buffer = Buffer.from('');
   let hasBeenCalled = false;
 
   stream.on('data', (chunk) => {