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/core/Axios.js

@@ -47,7 +47,7 @@ class Axios {
 
     config = mergeConfig(this.defaults, config);
 
-    const {transitional, paramsSerializer} = config;
+    const {transitional, paramsSerializer, headers} = config;
 
     if (transitional !== undefined) {
       validator.assertOptions(transitional, {
@@ -67,20 +67,22 @@ class Axios {
     // Set config.method
     config.method = (config.method || this.defaults.method || 'get').toLowerCase();
 
+    let contextHeaders;
+
     // Flatten headers
-    const defaultHeaders = config.headers && utils.merge(
-      config.headers.common,
-      config.headers[config.method]
+    contextHeaders = headers && utils.merge(
+      headers.common,
+      headers[config.method]
     );
 
-    defaultHeaders && utils.forEach(
+    contextHeaders && utils.forEach(
       ['delete', 'get', 'head', 'post', 'put', 'patch', 'common'],
-      function cleanHeaderConfig(method) {
-        delete config.headers[method];
+      (method) => {
+        delete headers[method];
       }
     );
 
-    config.headers = new AxiosHeaders(config.headers, defaultHeaders);
+    config.headers = AxiosHeaders.concat(contextHeaders, headers);
 
     // filter out skipped interceptors
     const requestInterceptorChain = [];