NCBC-324: ExecuteGet(string key, DateTime newExpiration) returns unexpected result codes

Change-Id: Ib9f6db0d5917490362f9c0124bdccce44b7d8da0
Reviewed-on: http://review.couchbase.org/30193
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Brett Lawson <brett19@gmail.com>