Support TypeScript import in browser (node-fetch#433)

Author: MarkHerhold

browser.js

@@ -1,4 +1,8 @@
 module.exports = exports = window.fetch;
+
+// Needed for TypeScript.
+exports.default = window.fetch;
+
 exports.Headers = window.Headers;
 exports.Request = window.Request;
 exports.Response = window.Response;