fix: use empty prototype

Author: GrosSacASac

src/Formidable.js

@@ -202,8 +202,8 @@ class IncomingForm extends EventEmitter {
       }
     }
     const callback = once(dezalgo(cb));
-    this.fields = {};
-    const files = {};
+    this.fields = Object.create(null);
+    const files = Object.create(null);
 
     this.on('field', (name, value) => {
       if (this.type === 'multipart' || this.type === 'urlencoded') {

src/plugins/json.js

@@ -27,7 +27,7 @@ function init(_self, _opts) {
   const parser = new JSONParser(this.options);
 
   parser.on('data', (fields) => {
-    this.fields = fields;
+    this.fields = Object.assign(Object.create(null), fields);
   });
 
   parser.once('end', () => {