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>

lib/helpers/buildURL.js

@@ -17,6 +17,7 @@ function encode(val) {
  *
  * @param {string} url The base of the url (e.g., http://www.google.com)
  * @param {object} [params] The params to be appended
+ * @param {?object} paramsSerializer
  * @returns {string} The formatted url
  */
 module.exports = function buildURL(url, params, paramsSerializer) {