Fix broken unit test

Author: vladsandu

tests/PostmarkClientSuppressionsTest.php

@@ -108,7 +108,7 @@ public function testInvalidMessageStreamThrowsException() {
         $emailAddress = "test-email@email.com";
         $suppressionChanges = array(new SuppressionChangeRequest($emailAddress));
 
-        $messageStream = "?invalid-stream?";
+        $messageStream = "invalid-stream-php-test";
 
         try {
             $result = $client->createSuppressions($suppressionChanges, $messageStream);