add_user can't make existing user readonly on MongoDB 2.6

Bug in unreleased PyMongo code. When talking with MongoDB 2.4, the following operations result in a read-only user:

db.add_user('jesse')

db.add_user('jesse', read_only=True)

But in MongoDB 2.6 the user still has the default role, "dbOwner", even after trying to make him read-only.