fix: allow passing a callback as paramsSerializer to buildURL (#6680)

* fix: allow passing a callback as paramsSerializer to buildURL

* fix: add missing semicolon

lib/helpers/buildURL.js

@@ -26,7 +26,7 @@ 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} options
+ * @param {?(object|Function)} options
  *
  * @returns {string} The formatted url
  */
@@ -38,6 +38,12 @@ export default function buildURL(url, params, options) {
   
   const _encode = options && options.encode || encode;
 
+  if (utils.isFunction(options)) {
+    options = {
+      serialize: options
+    };
+  } 
+
   const serializeFn = options && options.serialize;
 
   let serializedParams;