Do not treat is-encrypted as customer property

fixes #8977

Else for all files in a folder we would launch off more queries.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

apps/dav/lib/Connector/Sabre/CustomPropertiesBackend.php

@@ -52,6 +52,7 @@ class CustomPropertiesBackend implements BackendInterface {
 		'{http://owncloud.org/ns}downloadURL',
 		'{http://owncloud.org/ns}dDC',
 		'{http://owncloud.org/ns}size',
+		'{http://nextcloud.org/ns}is-encrypted',
 	);
 
 	/**