MB-100: correct typespec and silence dialyzer

Change-Id: I3d82a6e0a4bf0921f8aa5d957ad5718f4359895c
Reviewed-on: http://review.couchbase.org/24588
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>