Fix broken downloadBill (#23)

* use download bill api end point, and include buffer in return

* update demo accordingly

* typo

* remove default parameter as it's broke in node 4

demo/index.js

@@ -258,9 +258,9 @@ app.get('/download-bill', function (req, res) {
   const query = req.query;
   wx.payment.downloadBill(query.billDate, Payment.DOWNLOAD_BILL_TYPE.SUCCESS)
     .then(result => {
-      console.log('digest: ', result.digest);
-      if(result.data) {
-        result.data.pipe(res);
+      if(result.body) {
+        res.type('zip');
+        res.send(result.body);
       }
     })
     .catch(err => {

lib/Payment.js

@@ -415,7 +415,8 @@ class Payment {
     );
     return this.download(data, {
       gzip: !noGzip,
-    });
+      encoding: null, // get zip file as buffer
+    }, this.paymentUrls.DOWNLOAD_BILL);
   }
 
   /**
@@ -440,13 +441,14 @@ class Payment {
     return this.download(data, {
       agentOptions: this.getAgentOptions(),
       gzip: !noGzip,
-    });
+    }, this.paymentUrls.DOWNLOAD_FUND_FLOW);
   }
 
   /**
    * Download functionality
    * @param {object} data request data
    * @param {object=} requestOptions options send to request
+   * @param url to wechat api endpoint
    * @return {Promise} resolve or reject with:
    * {
    *   //an error object, rejected
@@ -460,15 +462,15 @@ class Payment {
    *   digest: 'SHA=ec45d7c24492dcd62d92472b0f2816c8d9a2d773',
    * }
    */
-  download(data, requestOptions) {
+  download(data, requestOptions, url) {
     return utils.buildXML(data).then(xmlData => {
       const myOptions = Object.assign(
         {
           strictSSL: false,
           json: false,
           gzip: true,
           method: 'POST',
-          url: this.paymentUrls.DOWNLOAD_FUND_FLOW,
+          url,
           body: xmlData,
         },
         requestOptions
@@ -502,6 +504,7 @@ class Payment {
           return resolve({
             //return the request stream
             data: stream,
+            body,
             digest: response.headers['digest'],
           });
         });