fix: configure buble to support object spread operator

Add objectAssign: 'Object.assign' option to buble plugin
to support ES6 object spread syntax.

Author: onikiienko

rollup.config.js

@@ -7,7 +7,11 @@ const config = {
     format: 'cjs',
     indent: false
   },
-  plugins: [buble()],
+  plugins: [
+    buble({
+      objectAssign: 'Object.assign',
+    }),
+  ],
 };
 
 export default config;