fix: replace Object.assign with _.extend

fixed #599

Author: leeyeh

src/adapter.js

@@ -1,3 +1,4 @@
+const _ = require('underscore');
 const adapters = {};
 
 const getAdapter = name => {
@@ -8,7 +9,7 @@ const getAdapter = name => {
   return adapter;
 };
 const setAdapters = newAdapters => {
-  Object.assign(adapters, newAdapters);
+  _.extend(adapters, newAdapters);
 };
 
 module.exports = {

src/uploader/s3.js

@@ -1,3 +1,4 @@
+const _ = require('underscore');
 const { ajax } = require('../utils');
 
 module.exports = function upload(uploadInfo, data, file, saveOptions = {}) {
@@ -18,7 +19,7 @@ module.exports = function upload(uploadInfo, data, file, saveOptions = {}) {
     url: uploadInfo.upload_url,
     method: 'PUT',
     data,
-    headers: Object.assign(
+    headers: _.extend(
       {
         'Content-Type': file.get('mime_type'),
         'Cache-Control': 'public, max-age=31536000',