Add tests for CAS collision

Couchbase::Bucket#cas failure condition that happens when a concurrent
writer modifies the document between the #get and #set calls inside
the #cas method.

Change-Id: I67294b1279fad9511bc74a0a5009482d46438b82
Reviewed-on: http://review.couchbase.org/30335
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>