fix failures of main.func_json --ps

in normal execution, the item is wrapped in Item_func_conv_charset.

in --ps the whole is wrapped again in Item_direct_ref_to_item

sql/item_jsonfunc.cc

@@ -1620,7 +1620,7 @@ static bool is_json_type(const Item *item)
     if (Type_handler_json_common::is_json_type_handler(item->type_handler()))
       return true;
     const Item_func_conv_charset *func;
-    if (!(func= dynamic_cast<const Item_func_conv_charset*>(item)))
+    if (!(func= dynamic_cast<const Item_func_conv_charset*>(item->real_item())))
       return false;
     item= func->arguments()[0];
   }