Fix HTTP headers default value

src/mixins/uploader-props.js

@@ -1,7 +1,7 @@
 export default {
   props: {
     filename  : { type: String, default: 'record'   },
-    headers   : { type: Object                    },
+    headers   : { type: Object, default: () => ({}) },
     uploadUrl : { type: String                      }
   }
 }