fix(AxiosHeaders): fix AxiosHeaders conversion to an object during config merging (#6243)

Co-authored-by: Dmitriy Mozgovoy <robotshara@gmail.com>

Author: petrovmiroslav

Diff for: lib/core/mergeConfig.js

@@ -3,7 +3,7 @@
 import utils from '../utils.js';
 import AxiosHeaders from "./AxiosHeaders.js";
 
-const headersToObject = (thing) => thing instanceof AxiosHeaders ? thing.toJSON() : thing;
+const headersToObject = (thing) => thing instanceof AxiosHeaders ? { ...thing } : thing;
 
 /**
  * Config-specific merge-function which creates a new config-object