Make sure error_log() always receives a string

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

lib/private/Log/File.php

@@ -145,6 +145,9 @@ class File implements IWriter, IFileBased {
 			error_log($entry);
 		}
 		if (php_sapi_name() === 'cli-server') {
+			if (!\is_string($message)) {
+				$message = json_encode($message);
+			}
 			error_log($message, 4);
 		}
 	}