ending a response when the last write was not flushed

might result in incomplete sends -> wait for the
drain event before ending the request.

lib/socket.io/transports/xhr-polling.js

@@ -72,8 +72,16 @@ Polling.prototype._write = function(message){
       if (this.request.headers.cookie) headers['Access-Control-Allow-Credentials'] = 'true';
     }
     this.response.writeHead(200, headers);
-    this.response.write(message);
-    this.response.end();
-    this._onClose();
+    var flushed = this.response.write(message);
+    if (flushed) {
+        this.response.end();
+        this._onClose();
+    } else {
+        var self = this;
+        this.response.connection.on("drain", function() {
+            self.response.end();            
+            self._onClose();
+        });
+    }
   }
 };