http: don't slice unless necessary

The incoming Buffer was being sliced just to possibly not be used.
Instead place the .slice inside the conditional to not be created unless
necessary.

lib/_http_server.js

@@ -352,11 +352,10 @@ function connectionListener(socket) {
       parser.finish();
       freeParser(parser, req);
 
-      // This is start + byteParsed
-      var bodyHead = d.slice(start + bytesParsed, end);
-
       var eventName = req.method === 'CONNECT' ? 'connect' : 'upgrade';
       if (EventEmitter.listenerCount(self, eventName) > 0) {
+        // This is start + byteParsed
+        var bodyHead = d.slice(start + bytesParsed, end);
         self.emit(eventName, req, req.socket, bodyHead);
       } else {
         // Got upgrade header or CONNECT method, but have no handler.