MB-100: do not send trailing zero with PLAIN auth request

Which only worked due to bug in couchbase sasl implementation.

Change-Id: I9ca5f65afb4181b9fbb71d940281c151601eb65c
Reviewed-on: http://review.couchbase.org/28335
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>