This change should also fix read preference support for mongos (see: `mongo_apply_mongos_rp()` in `cursor.c`). Previously, `primary preferred` or `secondary preferred` may have been passed as the `$readPreference` mode to mongos, which would have caused an assertion failure on the server side: https://github.com/mongodb/mongo/blob/master/src/mongo/client/dbclient_rs.cpp#L205