如果有前置cryptor，使用前置cryptor

Author: JacksonTian

lib/wechat.js

@@ -343,14 +343,17 @@ var serveEncrypt = function (that, req, res, next, _respond) {
   var timestamp = req.query.timestamp;
   var nonce = req.query.nonce;
 
+  // 判断是否已有前置cryptor
+  var cryptor = req.cryptor || that.cryptor;
+
   if (method === 'GET') {
     var echostr = req.query.echostr;
-    if (signature !== that.cryptor.getSignature(timestamp, nonce, echostr)) {
+    if (signature !== cryptor.getSignature(timestamp, nonce, echostr)) {
       res.writeHead(401);
       res.end('Invalid signature');
       return;
     }
-    var result = that.cryptor.decrypt(echostr);
+    var result = cryptor.decrypt(echostr);
     // TODO 检查appId的正确性
     res.writeHead(200);
     res.end(result.message);
@@ -372,12 +375,12 @@ var serveEncrypt = function (that, req, res, next, _respond) {
         }
         var xml = formatMessage(result.xml);
         var encryptMessage = xml.Encrypt;
-        if (signature !== that.cryptor.getSignature(timestamp, nonce, encryptMessage)) {
+        if (signature !== cryptor.getSignature(timestamp, nonce, encryptMessage)) {
           res.writeHead(401);
           res.end('Invalid signature');
           return;
         }
-        var decrypted = that.cryptor.decrypt(encryptMessage);
+        var decrypted = cryptor.decrypt(encryptMessage);
         var messageWrapXml = decrypted.message;
         if (messageWrapXml === '') {
           res.writeHead(401);