Fixed & Imporoved AxiosHeaders class (#5224)

* Refactored AxiosHeaders class;

* Added support for instances of AxiosHeaders as a value for the headers option;

Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/defaults/index.js

@@ -10,7 +10,7 @@ import formDataToJSON from '../helpers/formDataToJSON.js';
 import adapters from '../adapters/index.js';
 
 const DEFAULT_CONTENT_TYPE = {
-  'Content-Type': 'application/x-www-form-urlencoded'
+  'Content-Type': undefined
 };
 
 /**