better fetch-browser that preserves original fetch if it was there

Author: gobengo

fetch-browser.js

@@ -1,9 +1,13 @@
 // the whatwg-fetch polyfill installs the fetch() function
 // on the global object (window or self)
-//
-// Return that as the export for use in Webpack, Browserify etc.
+var fetchWasDefined = 'fetch' in global;
+var originalGlobalFetch = global.fetch;
+
 require('whatwg-fetch');
-var globalFetch = fetch;
-delete global.fetch;
+module.exports = fetch;
 
-module.exports = globalFetch;
+if (fetchWasDefined) {
+	global.fetch = originalGlobalFetch;
+} else {
+	delete global.fetch;
+}