Fix merging of params (#2656)

* Name function to avoid ESLint func-names warning

* Switch params config to merge list and update tests

* Restore testing of both false and null

* Restore test cases for keys without defaults

* Include test for non-object values that aren't false-y.

lib/core/enhanceError.js

@@ -20,7 +20,7 @@ module.exports = function enhanceError(error, config, code, request, response) {
   error.response = response;
   error.isAxiosError = true;
 
-  error.toJSON = function() {
+  error.toJSON = function toJSON() {
     return {
       // Standard
       message: this.message,