test: add Node unit tests for toFormData and refactor buildURL to avoid param reassignment (#7272)

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

lib/helpers/buildURL.js

@@ -29,29 +29,26 @@ function encode(val) {
  * @returns {string} The formatted url
  */
 export default function buildURL(url, params, options) {
-  /*eslint no-param-reassign:0*/
   if (!params) {
     return url;
   }
-  
+
   const _encode = options && options.encode || encode;
 
-  if (utils.isFunction(options)) {
-    options = {
-      serialize: options
-    };
-  } 
+  const _options = utils.isFunction(options) ? {
+    serialize: options
+  } : options;
 
-  const serializeFn = options && options.serialize;
+  const serializeFn = _options && _options.serialize;
 
   let serializedParams;
 
   if (serializeFn) {
-    serializedParams = serializeFn(params, options);
+    serializedParams = serializeFn(params, _options);
   } else {
     serializedParams = utils.isURLSearchParams(params) ?
       params.toString() :
-      new AxiosURLSearchParams(params, options).toString(_encode);
+      new AxiosURLSearchParams(params, _options).toString(_encode);
   }
 
   if (serializedParams) {