Can't list collections when mongos is running 3.0 and config servers are running 2.6 and auth is on

In 3.0 we introduced the listCollections command and an associated privilege to run it, replacing the old way of listing collections of querying system.namespaces directly. The problem is that mongoses load what privileges a user should have from the config servers, so if the config servers are still running 2.6, they will not provide the listCollections privilege. This means that future listCollections requests to a 3.0 mongos will fail with an "unauthorized" error.