Fixed FormData posting in browser environment by reverting #3785; (#4640)

* Fixed posting FormData in browser environment by reverting #3785;

* Added ability to keep `Content-Type` header in non-standard browser environments;

* Added browser test of FormData posting using external HTTP service (https://httpbin.org/post);

Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/adapters/xhr.js

@@ -27,6 +27,10 @@ module.exports = function xhrAdapter(config) {
       }
     }
 
+    if (utils.isFormData(requestData) && utils.isStandardBrowserEnv()) {
+      delete requestHeaders['Content-Type']; // Let the browser set it
+    }
+
     var request = new XMLHttpRequest();
 
     // HTTP basic authentication