Mark OembedAction, XrdAction, and (plugin) AutocompleteAction as read-only. Tweaked ApiStatusesShow and ApiTimelineUser to still claim read-only when hit with a HEAD request (usually link checkers or a precursor to a GET, and should be semantically equivalent to a GET without actually transferring data)

plugins/Autocomplete/autocomplete.php

@@ -165,4 +165,16 @@ class AutocompleteAction extends Action
             print json_encode($result) . "\n";
         }
     }
+
+    /**
+     * Is this action read-only?
+     *
+     * @param array $args other arguments
+     *
+     * @return boolean is read only action?
+     */
+    function isReadOnly($args)
+    {
+        return true;
+    }
 }