PluginBase->getResources() should only return files

Directories should not be returned. Previously it even returns resources\.. according to my test on Windows.

src/pocketmine/plugin/PluginBase.php

@@ -229,9 +229,11 @@ abstract class PluginBase implements Plugin{
 		$resources = [];
 		if(is_dir($this->file . "resources/")){
 			foreach(new \RecursiveIteratorIterator(new \RecursiveDirectoryIterator($this->file . "resources/")) as $resource){
+				if($resource->isFile()){
 					$resources[] = $resource;
 				}
 			}
+		}
 
 		return $resources;
 	}