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

test/specs/formdata.spec.js

@@ -0,0 +1,14 @@
+
+describe('FormData', function() {
+  it('should allow FormData posting', function () {
+    return axios.postForm('http://httpbin.org/post', {
+      a: 'foo',
+      b: 'bar'
+    }).then(({data}) => {
+      expect(data.form).toEqual({
+        a: 'foo',
+        b: 'bar'
+      });
+    });
+  });
+})