fix(headers): fixed support for setting multiple header values from an iterated source; (#6885)

test/specs/formdata.spec.js

@@ -1,13 +1,16 @@
+import {retryNetwork} from "../helpers/retry.js";
 
 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({
+  it('should allow FormData posting', async () => {
+    await retryNetwork(() => {
+      return axios.postForm('http://httpbin.org/post', {
         a: 'foo',
         b: 'bar'
+      }).then(({data}) => {
+        expect(data.form).toEqual({
+          a: 'foo',
+          b: 'bar'
+        });
       });
     });
   });