Don't allow to disable encryption via the API

Signed-off-by: Joas Schilling <coding@schilljs.com>

apps/provisioning_api/lib/Controller/AppConfigController.php

@@ -153,7 +153,7 @@ class AppConfigController extends OCSController {
 			throw new \InvalidArgumentException('The given key can not be set');
 		}
 
-		if ($app === 'core' && (strpos($key, 'public_') === 0 || strpos($key, 'remote_') === 0)) {
+		if ($app === 'core' && ($key === 'encryption_enabled' || strpos($key, 'public_') === 0 || strpos($key, 'remote_') === 0)) {
 			throw new \InvalidArgumentException('The given key can not be set');
 		}
 	}