added missing adapter default export

Author: mtrezza

src/index.js

@@ -2,8 +2,9 @@ import ApiMailAdapter from './ApiMailAdapter';
 import ApiPayloadConverter from './ApiPayloadConverter';
 import Errors from './Errors';
 
-module.exports = {
+export default ApiMailAdapter;
+export {
     ApiPayloadConverter,
     ApiMailAdapter,
-    Errors
+    Errors,
 };