fix(core): fixed config merging bug (axios#6668)

Co-authored-by: rana-aakash <[email protected]>
Co-authored-by: Dmitriy Mozgovoy <[email protected]>

lib/core/mergeConfig.js

@@ -19,7 +19,7 @@ export default function mergeConfig(config1, config2) {
   config2 = config2 || {};
   const config = {};
 
-  function getMergedValue(target, source, caseless) {
+  function getMergedValue(target, source, prop, caseless) {
     if (utils.isPlainObject(target) && utils.isPlainObject(source)) {
       return utils.merge.call({caseless}, target, source);
     } else if (utils.isPlainObject(source)) {
@@ -31,11 +31,11 @@ export default function mergeConfig(config1, config2) {
   }
 
   // eslint-disable-next-line consistent-return
-  function mergeDeepProperties(a, b, caseless) {
+  function mergeDeepProperties(a, b, prop , caseless) {
     if (!utils.isUndefined(b)) {
-      return getMergedValue(a, b, caseless);
+      return getMergedValue(a, b, prop , caseless);
     } else if (!utils.isUndefined(a)) {
-      return getMergedValue(undefined, a, caseless);
+      return getMergedValue(undefined, a, prop , caseless);
     }
   }
 
@@ -93,7 +93,7 @@ export default function mergeConfig(config1, config2) {
     socketPath: defaultToConfig2,
     responseEncoding: defaultToConfig2,
     validateStatus: mergeDirectKeys,
-    headers: (a, b) => mergeDeepProperties(headersToObject(a), headersToObject(b), true)
+    headers: (a, b , prop) => mergeDeepProperties(headersToObject(a), headersToObject(b),prop, true)
   };
 
   utils.forEach(Object.keys(Object.assign({}, config1, config2)), function computeConfigValue(prop) {