Update Sailthru/Sailthru/SailthruClient.cs

Updated a bug where ReceiveOptoutPost would return false no matter what (if optout exists in parameters, then it will fail if(!parameters.ContainsKey(key)), but if it doesn't exist in parameters, then it will fail if( parameters.Get("optout" == null))
Merge pull request #10 from sinneduy/patch-1

Update Sailthru/Sailthru/SailthruClient.cs