Bind fetch to window in the browser (node-fetch#434)

Author: MarkHerhold

browser.js

@@ -1,7 +1,7 @@
 module.exports = exports = window.fetch;
 
-// Needed for TypeScript.
-exports.default = window.fetch;
+// Needed for TypeScript and Webpack.
+exports.default = window.fetch.bind(window);
 
 exports.Headers = window.Headers;
 exports.Request = window.Request;