Added test + fix for Couchbase::Model#destroy

Fixes #29 and #30

Change-Id: I01158c7c8d12efb6f141286212bcd8fef50bec9e
Reviewed-on: http://review.couchbase.org/41320
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>