Handle cases where self doesn't exist

Similar to JakeChampion/fetch#253

fetch-npm-browserify.js

@@ -3,4 +3,5 @@
 //
 // Return that as the export for use in Webpack, Browserify etc.
 require('whatwg-fetch');
-module.exports = self.fetch.bind(self);
+var global = this.self || this;
+module.exports = global.fetch.bind(global);