fix "util.pump() is deprecated. Use readableStream.pipe()"

fix error introduced by b3988e3

src/node/utils/caching_middleware.js

@@ -168,7 +168,7 @@ CachingMiddleware.prototype = new function () {
         } else if (req.method == 'GET') {
           var readStream = fs.createReadStream(pathStr);
           res.writeHead(statusCode, headers);
-          readableStream.pipe(readStream, res);
+          readStream.pipe(res);
         } else {
           res.writeHead(statusCode, headers);
           res.end();