Release fetch resource

src/class/StatefulPromise.js

+// Copyright 2016 wkh237@github. All rights reserved.
+// Use of this source code is governed by a MIT-style license that can be
+// found in the LICENSE file.
+// @flow
+
+export default class StatefulPromise extends Promise {
+
+}

src/index.js

   RNFetchBlobStream,
   RNFetchBlobResponseInfo
 } from './types'
+import StatefulPromise from './class/StatefulPromise.js'
 import fs from './fs'
 import getUUID from './utils/uuid'
 import base64 from 'base-64'
       subscription.remove()
       subscriptionUpload.remove()
       stateEvent.remove()
+      delete promise['progress']
+      delete promise['uploadProgress']
+      delete promise['stateChange']
+      delete promise['cancel']
 
       if(err)
         reject(new Error(err, respInfo))