[fix] Fix undeterministic error in polling buffer processing (#529)

Ensure all assignments for chunks are Buffer when they should, even when onData is supposed to not
be called again.

Author: raulmt

lib/transports/polling.js

@@ -140,11 +140,10 @@ Polling.prototype.onDataRequest = function (req, res) {
   var self = this;
 
   function cleanup () {
-    chunks = isBinary ? new Buffer(0) : '';
     req.removeListener('data', onData);
     req.removeListener('end', onEnd);
     req.removeListener('close', onClose);
-    self.dataReq = self.dataRes = null;
+    self.dataReq = self.dataRes = chunks = null;
   }
 
   function onClose () {
@@ -163,7 +162,7 @@ Polling.prototype.onDataRequest = function (req, res) {
     }
 
     if (contentLength > self.maxHttpBufferSize) {
-      chunks = '';
+      chunks = isBinary ? new Buffer(0) : '';
       req.connection.destroy();
     }
   }