Add `self` to test browser helper

Most browsers have `self` as the `window` object, our test browser
doesn't have the `self` variable, recent update in form-data
form-data/form-data@f64a4c3 switched to use `self`, this fix adds
`self` reference back so it is aligned with browser behavior

test/helpers/browser.js

@@ -7,6 +7,7 @@ var vm = require('vm');
 
 function createContext() {
   var context = {
+    self: context,
     // required by browserify
     XMLHttpRequest: function() { throw new Error('not implemented'); },
     FormData: function() { throw new Error('not implemented'); },