Add method readAsStream on fs model

lib/utils/fs.js

@@ -157,6 +157,7 @@ module.exports = {
     statSync: fs.statSync,
     readdir: Promise.nfbind(fs.readdir),
     writeStream: writeStream,
+    readStream: fs.createReadStream,
     copy: Promise.nfbind(cp),
     copyDir: Promise.nfbind(cpr),
     tmpFile: genTmpFile,