Fixed toFormData regression bug (unreleased) with Array-like objects serialization; (#4714)

Added `toURLEncodedForm` helper;
Added automatic payload serialization to `application/x-www-form-urlencoded` to have parity with `multipart/form-data`;
Added test of handling `application/x-www-form-urlencoded` body by express.js;
Updated README.md;
Added missed param in JSDoc;
Fixed hrefs in README.md;

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

test/unit/adapters/http.js

@@ -12,8 +12,9 @@ var server, proxy;
 var AxiosError = require('../../../lib/core/AxiosError');
 var FormData = require('form-data');
 var formidable = require('formidable');
-const express = require('express');
-const multer = require('multer');
+var express = require('express');
+var multer = require('multer');
+var bodyParser = require('body-parser');
 
 describe('supports http with nodejs', function () {
 
@@ -1337,9 +1338,11 @@ describe('supports http with nodejs', function () {
         }).catch(done);
       });
     });
+
     describe('toFormData helper', function () {
       it('should properly serialize nested objects for parsing with multer.js (express.js)', function (done) {
-        const app = express();
+        var app = express();
+
         var obj = {
           arr1: ['1', '2', '3'],
           arr2: ['1', ['2'], '3'],
@@ -1374,4 +1377,35 @@ describe('supports http with nodejs', function () {
       });
     });
   });
+
+  describe('URLEncoded Form', function () {
+    it('should post object data as url-encoded form if content-type is application/x-www-form-urlencoded', function (done) {
+      var app = express();
+
+      var obj = {
+        arr1: ['1', '2', '3'],
+        arr2: ['1', ['2'], '3'],
+        obj: {x: '1', y: {z: '1'}},
+        users: [{name: 'Peter', surname: 'griffin'}, {name: 'Thomas', surname: 'Anderson'}]
+      };
+
+      app.use(bodyParser.urlencoded({ extended: true }));
+
+      app.post('/', function (req, res, next) {
+        res.send(JSON.stringify(req.body));
+      });
+
+      server = app.listen(3001, function () {
+        return axios.post('http://localhost:3001/', obj, {
+          headers: {
+            'content-type': 'application/x-www-form-urlencoded'
+          }
+        })
+          .then(function (res) {
+            assert.deepStrictEqual(res.data, obj);
+            done();
+          }, done);
+      });
+    });
+  });
 });