Fix arrow function in index.js

Author: wkh237

src/index.js

@@ -172,7 +172,7 @@ function fetch(...args:any):Promise {
       delete promise['uploadProgress']
       delete promise['stateChange']
       delete promise['cancel']
-      promise.cancel = () {
+      promise.cancel = () => {
         console.warn('finished request could not be canceled')
       }