an unnecessary E_NOTICE caused by CakeResponse::download()

Fix an unnecessary E_NOTICE when CakeResponse::download() is called without CakeResponse::compress().

ob_start() is called only if CakeResponse::compress() is called.
But, ob_flush() is always called in CakeResponse::_flushBuffer() if CakeResponse::download() is called.
It causes an unnecessary E_NOTICE and the following message is passed to ErrorHandler::handleError():

```
Notice:ob_flush() [<a href='http://php.net/ref.outcontrol'>ref.outcontrol</a>]: failed to flush buffer. No buffer to flush
```

This resolves it.