Worker: fixed quit() usually being useless

Worker::isRunning() returns false after it exits the run() function, even if tasks are stacked and it isn't shutdown.
While this is mostly harmless (since the internal destructor will join the thread anyway), this should be fixed anyway.

src/pocketmine/Worker.php

@@ -95,7 +95,7 @@ abstract class Worker extends \Worker{
 	public function quit(){
 		$this->isKilled = true;
 
-		if($this->isRunning()){
+		if(!$this->isShutdown()){
 			while($this->unstack() !== null);
 			$this->notify();
 			$this->shutdown();