Make a modifiable copy of the webdav properties in JS file list

Plugins can extend _getWebdavProperties to add custom properties.
These should not be added to the original properties list, so now the
FileList makes a copy of the array.

apps/files/js/filelist.js

@@ -1383,7 +1383,7 @@
 		 * Returns list of webdav properties to request
 		 */
 		_getWebdavProperties: function() {
-			return this.filesClient.getPropfindProperties();
+			return [].concat(this.filesClient.getPropfindProperties());
 		},
 
 		/**