Fix org-caldav: url-hexify-string lambda must accept optional allowed-chars arg

url-encode-url calls url-hexify-string with 2 args. Lambda only accepted 1
causing 'Wrong number of arguments' error. Add &optional allowed-chars.

config.el

@@ -1783,8 +1783,8 @@ current frame."
     "Preserve @ in UID path for Baikal CalDAV compatibility."
     (cl-letf* ((orig-hexify (symbol-function 'url-hexify-string))
                ((symbol-function 'url-hexify-string)
-                (lambda (str)
-                  (replace-regexp-in-string "%40" "@" (funcall orig-hexify str)))))
+                (lambda (str &optional allowed-chars)
+                  (replace-regexp-in-string "%40" "@" (funcall orig-hexify str allowed-chars)))))
       ad-do-it))
 
   ;; Fix #2: Some CalDAV events have nil SUMMARY or missing fields, causing