dav: fix wrong decoding of pluses in URLs

PHP's urldecode function does not decode URLs according to RFC 3986, but
according to the HTML 4.01 query parameter
encoding. This results in pluses being wrongly decoded to spaces even
though they should not be decoded at all.

Use rawurldecode instead, which follows RFC 3986 properly.

This fixes an issue where files on DAV shares containing pluses were
incorrectly decoded to spaces.

Fixes #15849

lib/private/Files/Storage/DAV.php

@@ -921,7 +921,7 @@ public function getDirectoryContent($directory): \Traversable {
 			}
 
 			foreach ($responses as $file => $response) {
-				$file = urldecode($file);
+				$file = rawurldecode($file);
 				$file = substr($file, strlen($this->root));
 				$file = $this->cleanPath($file);
 				$this->statCache->set($file, $response);