Refactor mergeConfig without utils.deepMerge (#2844)

* Adding failing test

* Fixing #2587 default custom config persisting

* Adding Concat keys and filter duplicates

* Fixed value from CPE

* update for review feedbacks

* no deepMerge

* only merge between plain objects

* fix rename

* always merge config by mergeConfig

* extract function mergeDeepProperties

* refactor mergeConfig with all keys, and add special logic for validateStatus

* add test for resetting headers

* add lots of tests and fix a bug

* should not inherit `data`

* use simple toString

* revert #1845

Co-authored-by: David Tanner <david.tanner@lifeomic.com>
Co-authored-by: Justin Beckwith <justin.beckwith@gmail.com>

lib/core/Axios.js

@@ -73,7 +73,7 @@ Axios.prototype.getUri = function getUri(config) {
 utils.forEach(['delete', 'get', 'head', 'options'], function forEachMethodNoData(method) {
   /*eslint func-names:0*/
   Axios.prototype[method] = function(url, config) {
-    return this.request(utils.merge(config || {}, {
+    return this.request(mergeConfig(config || {}, {
       method: method,
       url: url
     }));
@@ -83,7 +83,7 @@ utils.forEach(['delete', 'get', 'head', 'options'], function forEachMethodNoData
 utils.forEach(['post', 'put', 'patch'], function forEachMethodWithData(method) {
   /*eslint func-names:0*/
   Axios.prototype[method] = function(url, data, config) {
-    return this.request(utils.merge(config || {}, {
+    return this.request(mergeConfig(config || {}, {
       method: method,
       url: url,
       data: data