Update Observer.php

Fix issue : Notice: Undefined index: test in \magento\app\code\local\MJ\Customsmtp\Model\Observer.php on line 32

In that case I use array_key_exists, but you can check here the different ways if mine dont fit with your philosophy :
http://stackoverflow.com/questions/1966169/how-to-check-if-an-array-element-exists