fix(middleware): avoid using deprecated Buffer API

safe-buffer is already used and provides Buffer.alloc polyfill, so
just use Buffer.alloc directly to avoid hitting deprecated API.

Refs:
https://nodejs.org/api/deprecations.html#deprecations_dep0005_buffer_constructor

lib/middleware/common.js

@@ -58,7 +58,7 @@ var createServeFile = function (fs, directory, config) {
           return 206
         } else {
           // Multiple ranges are not supported. Maybe future?
-          responseData = new Buffer(0)
+          responseData = Buffer.alloc(0)
           return 416
         }
       }