Add spec for Buffer global

spec/fixtures/module/preload-node-off.js

@@ -1,6 +1,6 @@
 setImmediate(function () {
   try {
-    console.log([typeof process, typeof setImmediate, typeof global].join(' '))
+    console.log([typeof process, typeof setImmediate, typeof global, typeof Buffer].join(' '))
   } catch (e) {
     console.log(e.message)
   }

spec/fixtures/module/preload.js

@@ -1 +1 @@
-console.log([typeof require, typeof module, typeof process].join(' '))
+console.log([typeof require, typeof module, typeof process, typeof Buffer].join(' '))

spec/fixtures/module/send-later.js

@@ -1,4 +1,4 @@
 var ipcRenderer = require('electron').ipcRenderer
 window.onload = function () {
-  ipcRenderer.send('answer', typeof window.process)
+  ipcRenderer.send('answer', typeof window.process, typeof window.Buffer)
 }