NCBC-699: Upsert should take IDictionary input param

The whole IBucket interface has very clean definitions using interfaces
where appropriate. Upsert operation seems to stand out of the line there
with defining Dictionary input parameter instead of IDictionary. This
seemed as an oversight.

Change-Id: I6b5285fa90e7c60c49db0504ebe34b9139f0ea80
Reviewed-on: http://review.couchbase.org/42375
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>