[mod_webdav] ignore PROPFIND Depth for files

(thx meeb5)

ignore PROPFIND "Depth" request header for files (non-collections)

RFC4918 10.2. Depth Header
"If a resource does not have internal members, then the Depth header MUST be ignored."

x-ref:
  "Webdav + rclone backup"
  https://redmine.lighttpd.net/boards/2/topics/10081

src/mod_webdav.c

@@ -4014,9 +4014,8 @@ mod_webdav_propfind (request_st * const r, const plugin_config * const pconf)
         http_status_set_error(r, 403);
         return HANDLER_FINISHED;
     }
-    else if (0 != pb.depth) {
-        http_status_set_error(r, 403);
-        return HANDLER_FINISHED;
+    else {
+        pb.depth = 0;
     }
 
     pb.proplist.ptr  = NULL;