It raises InvalidUsername if username doesn't match bucket name

Currently cluster doesn't allow to change bucket username. Therefore
library allows it either to be nil or equal to name of the bucket.

This patch fixes only tests case to match behaviour

Change-Id: I4e39183f179c877cca5f890eda3446f4f75e4470
Reviewed-on: http://review.couchbase.org/28969
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>