add missing export (#5003)

lib/axios.js

@@ -14,6 +14,8 @@ import toFormData from './helpers/toFormData.js';
 import AxiosError from './core/AxiosError.js';
 import spread from './helpers/spread.js';
 import isAxiosError from './helpers/isAxiosError.js';
+import AxiosHeaders from './core/AxiosHeaders.js';
+
 
 /**
  * Create an instance of Axios
@@ -74,3 +76,11 @@ axios.formToJSON = thing => {
 };
 
 export default axios;
+
+export {
+  Axios,
+  AxiosHeaders,
+  AxiosError,
+  CanceledError,
+};
+